Metadata-Version: 1.1
Name: Djaq
Version: 0.1.1
Summary: A string-based Django query language
Home-page: https://github.com/paul-wolf/djaq
Author: Paul Wolf
Author-email: paul.wolf@yewleaf.com
License: MIT
Description: Djaq provides a query language for Django. In contrast to the QuerySet
        class that provides a Python API, Djaq queries are strings. A query
        string might look like this:
        
        ::
        
           (b.name as title, b.publisher.name as publisher) Book b
        
        This retrieves a list of book titles with book publisher. You can send
        Djaq queries from any language, Java, Javascript, golang, etc. to a
        Django application and get results as JSON.
        
        Of course, there is a common way to do this already by using frameworks
        like Django REST Framework (DRF), GraphQL, Django views, etc. The
        advantage of Djaq is you can immediately provide access without writing
        server side code except for security as explained below. Djaq is a good
        fit if you want:
        
        -  Microservice communication where some services don’t have access to
           the Django ORM or are not implemented with Python
        
        -  Fast local UI development
        
        -  Fast development of Proof of Concepts
        
        Djaq sits on top of the Django ORM. It can happily be used alongside
        QuerySets and sometimes calling a Djaq query even locally might be
        preferable to constructing a Queryset, although Djaq is not a
        replacement for QuerySets.
        
        Features you might appreciate:
        
        -  Immediate gratification with zero or minimal server-side code. There
           is minimal setup. And therefore, there is minimal wasted effort if
           you later move to another framework, like GraphQL or DRF.
        
        -  Djaq uses a syntax that lets you compose queries using Python-like
           expressions. The query format and syntax is chosen to be written by
           hand quickly. Readability is a key goal.
        
        -  Fast cursor semantics and explicit retrieval. It only gets data you
           asked for.
        
        -  Obvious performance behaviour. It will trigger a query in one obvious
           way through one of the generator methods: ``.dict()``, ``.tuples()``,
           ``.json()``.
        
        -  Complex expressions let you push computation down to the database
           layer from the client.
        
        -  A ready-to-go CRUD API that is easy to use. You can send requests to
           have an arbitrary number of Create, Read, Write, Delete operations
           done in a single request.
        
        -  Customisable behaviour using your own functions and data validators.
        
        -  A handy user interface for trying out queries on your data models.
        
        Djaq provides whitelisting of apps and models you want to expose. It
        also provides a simple permissions scheme via settings.
        
           Note that Djaq is still in an early phase of development. No
           warranties about reliability, security or that it will work exactly
           as described.
        
        .. figure:: bookshop/screenshots/djaq_ui.png?raw=true
           :alt: Djaq UI
        
           Djaq UI
        
        Limitations
        -----------
        
        Compared to other frameworks like GraphQL and DRF, you can’t easily
        implement complex business rules on the server. This might be a deal
        breaker for your application. In particular, if you want to restrict
        access from untrusted clients to prevent them accessing some rows of
        your DB, this is more work than just installing Djaq. In that case, you
        might look at one of those other solutions or Plain Old Django Views.
        
        Djaq only supports Postgresql at this time.
        
        Values in the ``choices`` argument of fields that take only a limited
        set of values will not be retrieved. Instead you get the raw field
        value. These are not accessible to Djaq which only retrieves what is
        available via SQL.
        
        Quickstart
        ----------
        
        You need Python 3.6 or higher and Django 2.1 or higher.
        
        Install:
        
        ::
        
           pip install Djaq
        
        The bleeding edge experience:
        
        ::
        
           pip install https://github.com/paul-wolf/djaq/archive/master.zip
        
        Use:
        
        .. code:: python
        
           from djaq.query import DjangoQuery as DQ
        
           print(list(DQ("(b.name as title, b.publisher.name as publisher) Book b").dicts()))
        
           [{'title': 'Name grow along.', 'publisher': 'Long, Lewis and Wright'}, {'title': 'We pay single record.', 'publisher': 'Long, Lewis and Wright'}, {'title': 'Natural develop available manager.', 'publisher': 'Long, Lewis and Wright'}, {'title': 'Fight task international.', 'publisher': 'Long, Lewis and Wright'}, {'title': 'Discover floor phone.', 'publisher': 'Long, Lewis and Wright'}]
        
        Providing an API
        ----------------
        
        We’ll assume below you are installing the Djaq UI. This is not required
        to provide an API but is very useful to try things out.
        
        Install the API and UI in settings:
        
        .. code:: python
        
           INSTALLED_APPS = (
               ...
               djaq.djaq_api,
               djaq.djaq_ui,
           )
        
        Configure urls in urls.py:
        
        .. code:: python
        
           urlpatterns = [
               ...
               path("dquery/", include("djaq.djaq_ui.urls")),`
               path("djaq/", include("djaq.djaq_api.urls")),`
           ]
        
        You are done. You can start sending requests to:
        
        ::
        
           /djaq/api/request/
        
        The UI will be available at:
        
        ::
        
           /dquery
        
        Note the UI will send requests to the API endpoint so will not work
        without that being configured. You send a request in this form to the
        api endpoint:
        
        .. code:: python
        
           {
            "queries": [
             {
              "q": "(b.id,b.name,b.pages,b.price,b.rating,b.publisher,b.alt_publisher,b.pubdate,b.in_print,) books.Book b",
              "context": {},
              "limit": "100",
              "offset": "0"
             }
            ]
           }
        
        The UI will create this JSON for you if you want to avoid typing it.
        
        You can also create objects, update them and delete them:
        
        .. code:: python
        
           {
              "queries": [
               {
                 "q": "(b.id,b.name,b.pages,b.price,b.rating,b.publisher,b.alt_publisher,b.pubdate,b.in_print,) books.Book b",
                 "context": {},
                 "limit": "100",
                 "offset": "0"
               }
              ],
             "creates":[{
                "_model":"Book"
                "name":"my new book",
                }],
             "updates":[{
                "_model":"Book"
                "_pk": 37,
                "name":"my new title",
                }],
             "deletes":[{
                "_model":"Book"
                  "_pk": 37,
                }]
           }
        
        You can send multiple ``queries``, ``creates``, ``updates``, ``deletes``
        operations in a single request.
        
        Settings
        --------
        
        The API and UI will use two settings:
        
        -  DJAQ_WHITELIST: a list of apps/models that the user is permitted to
           include in queries.
        
        -  DJAQ_PERMISSIONS: permissions required for staff and superuser.
        
        In the following example, we allow the models from ‘books’ to be exposed
        as well as the ``User`` model. We also require the caller to be both a
        staff member and superuser:
        
        ::
        
           DJAQ_WHITELIST = {
               "django.contrib.auth": ["User"],
               "books": [
                   "Profile",
                   "Author",
                   "Consortium",
                   "Publisher",
                   "Book_authors",
                   "Book",
                   "Store_books",
                   "Store",
               ],
           }
           DJAQ_UI_URL = None
           DJAQ_API_URL = None
           DJAQ_PERMISSIONS = {
               "creates": True,
               "updates": True,
               "deletes": True,
               "staff": True,
               "superuser": True,
           }
        
        If we want to allow all models for an app, we can leave away the list of
        models. This will have the same effect as the setting above.
        
        ::
        
           DJAQ_WHITELIST = {
               "django.contrib.auth": ["User"],
               "books": [],
           }
        
        For permissions, you can optionally require any requesting user to be
        staff and/or superuser. And you can deny or allow update operations. If
        you do not provide explicit permissions for update operations, the API
        will respond with 401 if one of those operations is attempted.
        
        Custom API
        ----------
        
        You can write your own custom API endpoint. Here is what a view function
        for your data layer might look like with Djaq:
        
        .. code:: python
        
           @login_required
           def djaq_view(request):
               data = json.loads(request.body.decode("utf-8"))
               query_string = data.get("q")
               offset = int(data.get("offset", 0))
               limit = int(data.get("limit", 0))
               context = data.get("context", {})
               return JsonResponse(
                   {
                      "result": list(
                          DQ(query_string)
                          .context(context)
                          .limit(limit)
                          .offset(offset)
                          .dicts()
                      )
                   }
               )
        
        You can now query any models in your entire Django deployment remotely,
        provided the authentication underlying the ``login_required`` is
        satisfied. This is a good solution if your endpoint is only available to
        trusted clients who hold a valid authentication token or to clients
        without authentication who are in your own network and over which you
        have complete control. It is a bad solution on its own for any public
        access since it exposes Django framework models, like users,
        permissions, etc.
        
        Most likely you want to control access in two ways:
        
        -  Allow access to only some apps/models
        
        -  Allow access to only some rows in each table and possibly only some
           fields.
        
        For controlling access to models, use the whitelist parameter in
        constructing the DjangoQuery:
        
        .. code:: python
        
           DQ(query_string, whitelist={"books": ["Book", "Publisher",],})
             .context(context)
             .limit(limit)
             .offset(offset)
             .dicts()
        
        This restricts access to only the ``book`` app models, Book and Publish.
        
        You probably need a couple more things if you want to expose this to a
        browser. But this gives an idea of what you can do. The caller now has
        access to any authorised model resource. Serialisation is all taken care
        of. Djaq comes already with a view similar to the above. You can just
        start calling and retrieving any data you wish. It’s an instant API to
        your application provided you trust the client or have sufficient access
        control in place.
        
        Performance
        -----------
        
        Once the query is parsed, it is about the same overhead as calling this:
        
        ::
        
           conn = connections['default']
           cursor = conn.cursor()
           self.cursor = self.connection.cursor()
           self.cursor.execute(sql)
        
        Parsing is pretty fast:
        
        ::
        
           In [12]: %timeit list(DQ("(b.name) Book{ilike(b.name, 'A%')} b").parse())
           314 µs ± 4.1 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
        
        and might be a negligible factor if you are parsing during a remote call
        as part of a view function.
        
        But if you want to iterate over, say, a dictionary of variables locally,
        you’ll want to parse once:
        
        .. code:: python
        
           dq = DQ("(b.name) Book{ilike(b.name, '$(namestart)')} b")
           dq.parse()
           for vars in var_list:
               results = list(dq.context(vars).tuples())
               <do something with results>
        
        Note that each call of ``context()`` causes the cursor to execute again
        when ``tuples()`` is iterated.
        
        Query usage guide
        -----------------
        
        Throughout, we use models somewhat like those from Django’s bookshop
        example:
        
        .. code:: python
        
           from django.db import models
        
           class Author(models.Model):
               name = models.CharField(max_length=100)
               age = models.IntegerField()
        
           class Publisher(models.Model):
               name = models.CharField(max_length=300)
        
           class Book(models.Model):
               name = models.CharField(max_length=300)
               pages = models.IntegerField()
               price = models.DecimalField(max_digits=10, decimal_places=2)
               rating = models.FloatField()
               authors = models.ManyToManyField(Author)
               publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)
               pubdate = models.DateField()
        
           class Store(models.Model):
               name = models.CharField(max_length=300)
               books = models.ManyToManyField(Book)
        
        These examples use auto-generated titles and names and we have a
        slightly more complicated set of models than shown above.
        
        Let’s get book title (name), price, discounted price, amount of discount
        and publisher name wherever the price is over 50.
        
        .. code:: python
        
           result = \
             list(DQ("""(b.name,
                  b.price as price,
                  0.2 as discount,
                  b.price * 0.2 as discount_price,
                  b.price - (b.price*0.2) as diff,
                  Publisher.name
                 ) Book{b.price > 50} b""").dicts())
        
        ``result`` now contains a list of dicts each of which is a row in the
        result set. One example:
        
        .. code:: python
        
           [{'b_name': 'Address such conference.',
             'price': Decimal('99.01'),
             'discount': Decimal('0.2'),
             'discount_price': Decimal('19.802'),
             'diff': Decimal('79.208'),
             'publisher_name': 'Arnold Inc'}]
        
        Here is the structure of the syntax:
        
        ::
        
           (<field_exp1>, ...) <ModelName>{<filter_expression>} <alias> order by (<field_exp1>, ...)
        
        Whitespace does not matter too much. You could put things on separate
        lines:
        
        .. code:: python
        
           (
              b.name, b.price,
              Publisher.name
           )
           Book{p.price > 50} b
        
        Always start with column expressions you want to return in parens:
        
        ::
        
           (b.name, b.price, Publisher.name)
        
        These expressions can be Django Model fields or arithmetic expressions
        or any expression supported by functions of your underlying database.
        Columns are automatically given names. But you can give them your own
        name:
        
        ::
        
           (b.name as title, b.price as price, Publisher.name as publisher)
        
        Next is the model alias declaration:
        
        ::
        
           Book b
        
        or if we want to filter and get only books over 50 in price:
        
        ::
        
           Book{b.price > 50} b
        
        ``Book`` is the Django Model name. ``b`` is an alias we can use as an
        abbreviation in the filter or returned column expressions. We put the
        filter in curly braces, ``{}``, between the model name and alias. Use
        Python syntax to express the filter. Also use Python syntax to express
        the data to return. You don’t have access to the Python Standard
        Library. This is basically the intersection of SQL and Python:
        
        The following filter:
        
        ::
        
           {b.price > 50 and ilike(Publisher.name, 'A%')}
        
        will be translated to SQL:
        
        ::
        
           b.price > 50 AND publisher.name ILIKE 'A%'
        
        The expressions are fully parsed so they are not subject to SQL
        injection. Trying to do so will cause an exception.
        
        You might notice in the above examples, Publisher does not use an alias.
        If you wanted an alias for Publisher, you could use a more complicated
        syntax:
        
        ::
        
           (b.name, b.price) Book b
           -> (p.name) Publisher.name p
        
        Notice, we use the ``->`` symbol to add another aliased relationship.
        This is one of three options: ``->``, ``<-``, ``<>`` that indicate you
        want to explicitly join via an SQL LEFT, RIGHT or INNER join
        respectively. But you don’t need to do this. LEFT joins will always be
        implicit. We did not even need to refer to the Publisher model directly.
        We could have done this:
        
        ::
        
           (b.name, b.price, b.publisher.name as publisher)
           Book{p.price > 50} b
        
        Our example model also has an owner model called “Consortium” that is
        the owner of the publisher:
        
        .. code:: python
        
           In [16]: print(list(DQ("(b.name, b.price, b.publisher.name, b.publisher.owner.name) Book b").limit(1).dicts()))
           Out[16]: [{'b_name': 'Range total author impact.', 'b_price': Decimal('12.00'), 'b_publisher_name': 'Wright, Taylor and Fitzpatrick', 'b_publisher_owner_name': 'Publishers Group'}]
        
        To recap, there are three alternative patterns to follow to get the
        publisher name in the result set:
        
        .. code:: python
        
           In [13]: print(list(DQ("(b.name, b.price) Book b -> (p.name)Publisher p").limit(1).dicts()))
        
           In [14]: print(list(DQ("(b.name, b.price, Publisher.name) Book b").limit(1).dicts()))
        
           In [15]: print(list(DQ("(b.name, b.price, b.publisher.name) Book b").limit(1).dicts()))
        
        Note that the above will each produce slightly different auto-generated
        output names unless you provide your own aliases.
        
        Signal that you want to summarise results using an aggregate function:
        
        .. code:: python
        
           list(DQ("(b.publisher.name as publisher, count(b.id) as book_count) Book b").dicts())
        
           [
               {
                   "publisher": "Martinez, Clark and Banks",
                   "book_count": 6
               },
               {
                   "publisher": "Fischer-Casey",
                   "book_count": 9
               },
               etc.
           ]
        
        Order by name:
        
        ::
        
           (b.name, b.price, b.publisher.name as publisher)
           Book{p.price > 50} b
           order by b.name
        
        Get average, minimum and maximum prices:
        
        .. code:: python
        
           list(DQ("(avg(b.price) as average, min(b.price) as minimum, max(b.price) as maximum) Book b).dicts())
           [
              {
                 "average": "18.5287169247794985",
                 "minimum": "3.00",
                 "maximum": "99.01"
              }
           ]
        
        Count all books:
        
        .. code:: python
        
           list(DQ("(count(b.id)) Book b").dicts())
        
           [
               {
                   "countb_id": 149999
               }
           ]
        
        You can qualify model names with the app name or registered app path:
        
        ::
        
           (b.name, b.publisher.name) books.Book b
        
        You’ll need this if you have models from different apps with the same
        name.
        
        To pass parameters, use variables in your query, like ``'$(myvar)'``:
        
        .. code:: python
        
           In [30]: oldest = '2018-12-20'
               ...: list(DQ("(b.name, b.pubdate) Book{b.pubdate >= '$(oldest)'} b").context({"oldest": oldest}).limit(5).tuples())
           Out[30]:
           [('Available exactly blood.', datetime.date(2018, 12, 20)),
            ('Indicate Congress none always.', datetime.date(2018, 12, 24)),
            ('Old beautiful three program.', datetime.date(2018, 12, 25)),
            ('Oil onto mission.', datetime.date(2018, 12, 21)),
            ('Key same effect me.', datetime.date(2018, 12, 23))]
        
        Notice that the variable holder, ``$()``, *must* be in single quotes.
        
        Query UI
        ~~~~~~~~
        
        You can optionally install a query user interface to try out queries on
        your own data set:
        
        -  After installing djaq, add ``djaq.djaq_ui`` to INSTALLED_APPS
        
        -  Add ``path("dquery/", include("djaq.djaq_ui.urls")),`` to
           ``urlpattenrs   in the sites``\ urls.py\`
        
        Navigate to \`/dquery/’ in your app and you should be able to try out
        queries.
        
        Functions
        ---------
        
        If a function is not defined by DjangoQuery, then the function name is
        passed without further intervention to the underlying SQL. A user can
        define new functions at any time by adding to the custom functions.
        Here’s an example of adding a regex matching function:
        
        .. code:: python
        
           DjangoQuery.functions["REGEX"] = "{} ~ {}"
        
        Now find all book names starting with ‘B’:
        
        .. code:: python
        
           DQ("(b.name) Book{regex(b.name, 'B.*')} b")
        
        We always want to use upper case for the function name when defining the
        function. Usage of a function is then case-insensitive. You may wish to
        make sure you are not over-writing existing functions. “REGEX” already
        exists, for instance.
        
        You can also provide a ``callable`` to ``DjangoQuery.functions``. The
        callable needs to take two arguments: the function name and a list of
        positional parameters and it must return SQL as a string that can either
        represent a column expression or some value expression from the
        underlying backend.
        
        In the following:
        
        .. code:: python
        
           DQ("(b.name) Book{like(upper(b.name), upper('$(name_search)'))} b")
        
        ``like()`` is a Djaq-defined function that is converted to
        ``field LIKE string``. Whereas ``upper()`` is sent to the underlying
        database because it’s a common SQL function. Any function can be created
        or existing functions mutated by updating the ``DjangoQuery.functions``
        dict where the key is the upper case function name and the value is a
        template string with ``{}`` placeholders. Arguments are positionally
        interpolated.
        
        Above, we provided this example:
        
        .. code:: python
        
           DQ("""(
              sum(iif(b.rating < 5, b.rating, 0)) as below_5,
              sum(iif(b.rating >= 5, b.rating, 0)) as above_5
           ) Book b""")
        
        We can simplify further by creating a new function. The IIF function is
        defined like this:
        
        ::
        
           "CASE WHEN {} THEN {} ELSE {} END"
        
        We can create a ``SUMIF`` function like this:
        
        ::
        
           DjangoQuery.functions['SUMIF'] = "SUM(CASE WHEN {} THEN {} ELSE {} END)"
        
        Now we can rewrite the above like this:
        
        .. code:: python
        
           DQ("""(
               sumif(b.rating < 5, b.rating, 0) as below_5,
               sumif(b.rating >= 5, b.rating, 0) as above_5
               ) Book b""")
        
        Here’s an example providing a function:
        
        .. code:: python
        
           def concat(funcname, args):
               """Return args spliced by sql concat operator."""
               return " || ".join(args)
        
           DjangoQuery.functions['CONCAT'] = concat
        
        Parameters
        ----------
        
        We call the Django connection cursor approximately like this:
        
        .. code:: python
        
           from django.db import connections
           cursor = connections['default']
           cursor.execute(sql, context_dict)
        
        When we execute the resulting SQL query, named parameters are used. You
        *must* name your parameters. Positional parameters are not passed:
        
        .. code:: python
        
           oldest = '2000-01-01'
           DQ("(b.id) Book{b.pub_date >= '$(oldest)'} b").context({"oldest": oldest}).tuples()
        
        Notice that any parameterised value must be represented in the query
        expression in single quotes:
        
        ::
        
           '$(myparam)'
        
        Therefore, when you add subqueries, their parameters have to be supplied
        at the same time.
        
        Note what is happening here:
        
        ::
        
           name_search = 'Bar.*'
           DQ("(b.id) Book{regex(b.name, '%(name_search)')} b").context(locals()).tuples()
        
        To get all books starting with ‘Bar’. Or:
        
        .. code:: python
        
           DQ("(b.name) Book{like(upper(b.name), upper('$(name_search)'))} b").context(request.POST)
        
        Provided that ``request.POST`` has a ``name_search`` key/value.
        
        You can provide a validation class that will return context variables.
        The default class used is called ``ContextValidator()``. You can
        override this to provide a validator that raises exceptions if data is
        not valid or mutates the context data, like coercing types from ``str``
        to ``int``:
        
        .. code:: python
        
           class MyContextValidator(ContextValidator):
               def get(self, key, value):
                   if key == 'order_no':
                       return int(value)
                   return value
        
               def context(self):
                   if not 'order_no' in self.data:
                       raise Exception("Need order no")
                   self.super().context()
        
        Then add the validator:
        
        .. code:: python
        
           order_no = "12345"
           DQ("(o.order_no, o.customer) Orders{o.order_no == '%(order_no)')} b")
               .validator(MyContextValidator)
               .context(locals())
               .tuples()
        
        You can set your own validator class in Django settings:
        
        ::
        
           DJAQ_VALIDATOR = MyValidator
        
        The ``request`` parameter of the API view is added to the context and
        will be available to the validator as ``request``.
        
        Column expressions
        ------------------
        
        Doing column arithmetic is supported directly in the query syntax:
        
        .. code:: python
        
           DQ("""(b.name,
               b.price as price,
               0.2 as discount,
               b.price*0.2 as discount_price,
               b.price - (b.price*0.2) as diff
               ) Book b""")
        
        You can use constants:
        
        .. code:: python
        
           In [60]: list(DQ("(b.name, 'great read') Book b").limit(1).tuples())
           Out[60]: [('Range total author impact.', 'great read')]
        
        You can use the common operators and functions of your underlying db.
        
        The usual arithmetic:
        
        .. code:: python
        
           In [36]: list(DQ("(b.name, 1+1) Book b").limit(1).tuples())
           Out[36]: [('Range total author impact.', 2)]
           In [38]: list(DQ("(b.name, 2.0/4) Book b").limit(1).tuples())
           Out[38]: [('Range total author impact.', Decimal('0.50000000000000000000'))]
           In [44]: list(DQ("(2*3) Book b").limit(1).tuples())
           Out[44]: [(6,)]
        
        Modulo:
        
        .. code:: python
        
           In [55]: list(DQ("(mod(4.0,3)) Book b").limit(1).tuples())
           Out[55]: [(Decimal('1.0'),)]
        
        Comparison as a boolean expression:
        
        .. code:: python
        
           In [45]: list(DQ("(2 > 3) Book b").limit(1).tuples())
           Out[45]: [(False,)]
        
        While the syntax has a superficial resemblance to Python, you do not
        have access to any functions of the Python Standard Libary.
        
        Subqueries and ``in`` clause
        ----------------------------
        
        You can reference subqueries within a Djaq expression using
        
        -  Another DjangoQuery
        -  A Queryset
        -  A list
        
        The two most useful cases are using a subquery in the filter condition:
        
        ::
        
           DQ('(b.id, b.name) Book{b.id in ["(Book.id)"]} b')
        
        And using a subquery in the selected columns expression:
        
        ::
        
           DQ('(p.name, ["(count(b.id)) Book{Publisher.id == b.publisher} b"]) Publisher p')
        
        You can use an IN clause with the keyword ``in`` (note lower case) If
        you are writing queries via the Python API. Create one DjangoQuery and
        reference it with ``@queryname``:
        
        ::
        
           DQ("(b.id) Book{name == 'B*'} b", name='dq_sub')
           dq = DQ("(b.name, b.price) Book{id in '@dq_sub'} b")
        
        Note that you have to pass a name to the DjangoQuery to reference it
        later. We can also use the ``data`` parameter to pass a QuerySet to the
        DjangoQuery:
        
        ::
        
           qs = Book.objects.filter(name__startswith="B").only('id')
           dq = DQ("(b.name, b.price) Book{id in '@qs_sub'} b", names={"qs_sub": qs})
        
           qs = Book.objects.filter(name__startswith="B").only('id')
           ids = [rec.id for rec in qs]
           dq = DQ("(b.name, b.price) Book{id in '@qs_sub'} b", names={"qs_sub": ids})
        
        As with QuerySets it is nearly always faster to generate a sub query
        than use an itemised list.
        
        Order by
        --------
        
        You can order_by like this:
        
        ::
        
           DQ("(b.id) Book{b.price > 20} b order by (b.name)")
        
        Descending order:
        
        ::
        
           DQ("(b.id) Book{b.price > 20} b order by (-b.name)")
        
        You can have multple order by expressions.
        
        ::
        
           DQ("(b.name, Publisher.name) Book{b.price > 20} b order by (-b.name, b.publisher.name)")
        
        Count
        -----
        
        There are a couple ways to count results. These both return the exact
        same thing:
        
        ::
        
           DQ("(Book.id)").count()
        
           DQ("(count(Book.id)) Book").value()
        
        Datetimes
        ---------
        
        Datetimes are provided as strings in the iso format that your backend
        expects, like ‘2019-01-01 18:00:00’.
        
        Constants
        ---------
        
        ``None``, ``True``, ``False`` are replaced in SQL with ``NULL``,
        ``TRUE``, ``FALSE``. All of the following work:
        
        ::
        
           DQ("(b.id, b.name) Book{in_print is True} b")
           DQ("(b.id, b.name) Book{in_print is not True} b")
           DQ("(b.id, b.name) Book{in_print is False} b")
           DQ("(b.id, b.name) Book{in_print == True} b")
        
        Slicing
        -------
        
        You cannot slice a DjangoQuery because this would frustrate a design
        goal of Djaq to provide the performance advantages of cursor-like
        behaviour.
        
        You can use ``limit()`` and ``offset()``:
        
        ::
        
           DjangoQuery("...").offset(1000).limit(100).tuples()
        
        Which will provide you with the first hundred results starting from the
        1000th record.
        
        Rewind cursor
        -------------
        
        You can rewind the cursor but this is just executing the SQL again:
        
        ::
        
           list(dq.tuples())
        
           # now, calling `dq.tuples()` returns nothing
        
           list(dq.rewind().tuples())
        
           # you will again see results
        
        If you call ``DjangoQuery.context(data)``, that will effectively rewind
        the cursor since an entirely new query is created and the implementation
        currently doesn’t care if ``data`` is the same context as previously
        supplied.
        
        Schema
        ------
        
        There is a function to get the schema available to a calling client:
        
        .. code:: python
        
           from djaq.app_utils import get_schema
           print(get_schema())
        
        Pass the same whitelist you use for exposing the query endpoint:
        
        .. code:: python
        
           wl = {"books": []}
           print(get_schema(whitelist=wl))
        
        Comparing to Django QuerySets
        -----------------------------
        
        Djaq queries can be sent over the wire as a string. That is the main
        difference with Quersets. Even then, Djaq is not a replacement for
        Querysets. Querysets are highly integrated with Django and have been
        developed over 15 years by many developers. It is a very well thought
        out framework that is the best choice working within a service based on
        Django’s ORM. This section is intended to highlight differences for
        users with high familiarity with the ``QuerySet`` class for the purpose
        of understanding limitations and capabilities of DjangoQuery.
        
        Django provides significant options for adjusting query generation to
        fit a specific use case, ``only()``, ``select_related()``,
        ``prefetch_related()`` are all useful for different cases. Here’s a
        point-by-point comparison with Djaq:
        
        -  ``only()``: Djaq always works in “only” mode. Only explicitly
           requested fields are returned.
        
        -  ``select_related()``: The select clause only returns those columns
           explicitly defined. This feature makes loading of related fields
           non-lazy. In contrast, queries are always non-lazy in Djaq.
        
        -  ``prefetch_related()``: When you have a m2m field as a column
           expression, the model hosting that field is repeated in results as
           many times as necessary. Another way is to use a separate query for
           the m2m related records. In anycase, this is not required in Djaq.
        
        -  F expressions: These are workarounds for not being able to write
           expressions in the query for things like column value arithmetic and
           other expressions you want to have the db calculate. Djaq lets you
           write these directly and naturally as part of its syntax.
        
        -  To aggregate with Querysets, you use ``aggregate()``, whereas Djaq
           aggregates results whenever an aggregate function appears in the
           column expressions.
        
        -  Model instances with QuerySets exactly represent the corresponding
           Django model. Djaq’s usual return formats, like ``dicts()``,
           ``tuples()``, etc. are more akin to ``QuerySet.value_list()``.
        
        Let’s look at some direct query comparisons:
        
        Get the average price of books:
        
        ::
        
           DQ("(avg(b.price)) Book b")
        
        compared to QuerySet:
        
        ::
        
           Book.objects.all().aggregate(Avg('price'))
        
        Get the difference off the maximum price:
        
        ::
        
           DQ("(Publisher.name, max(Book.price) - avg(Book.price) as price_diff) Book b")
        
        compared to QuerySet:
        
        ::
        
           Book.objects.aggregate(price_diff=Max('price', output_field=FloatField()) - Avg('price'))
        
        Count books per publisher:
        
        ::
        
           DQ("(Publisher.name, count(Book.id) as num_books) Book b")
        
        compared to QuerySet:
        
        ::
        
           Publisher.objects.annotate(num_books=Count("book"))
        
        Count books with ratings up to and over 5:
        
        .. code:: python
        
           DQ("""(sum(iif(b.rating < 5, b.rating, 0)) as below_5,
               sum(iif(b.rating >= 5, b.rating, 0)) as above_5)
               Book b""")
        
        compared to QuerySet:
        
        .. code:: python
        
           above_5 = Count('book', filter=Q(book__rating__gt=5))
           below_5 = Count('book', filter=Q(book__rating__lte=5))
           Publisher.objects.annotate(below_5=below_5).annotate(above_5=above_5)
        
        Get average, maximum, minimum price of books:
        
        .. code:: python
        
           DQ("(avg(b.price), max(b.price), min(b.price)) Book b")
        
        compared to QuerySet:
        
        ::
        
           Book.objects.aggregate(Avg('price'), Max('price'), Min('price'))
        
        Just as there is a ModelInstance class in Django, we have a DQResult
        class:
        
        ``objs()``: return a DQResult for each result row, basically a namespace
        for the object:
        
        .. code:: python
        
           dq = DQ("(b.id, b.name, Publisher.name as publisher) Book b")
           for book in dq.objs():
               title = book.name
               publisher = book.publisher
               ...
        
        Note that by default, you iterate using a generator. You cannot slice a
        generator.
        
        Simple counts:
        
        ``DjangoQuery.value()``: when you know the result is a single row with a
        single value, you can immediately access it without further iterations:
        
        .. code:: python
        
           DQ("(count(b.id)) Book b").value()
        
        will return a single integer value representing the count of books.
        
        Django Subquery and OuterRef
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        
        The following do pretty much the same thing:
        
        ::
        
           # QuerySet
           pubs = Publisher.objects.filter(pk=OuterRef('publisher')).only('pk')
           Book.objects.filter(publisher__in=Subquery(pubs))
        
           # Djaq
           DQ("(p.id) Publisher p", name='pubs')
           DQ("(b.name) Book{publisher in '@pubs'} b")
        
        Obviously, in both cases, you would be filtering Publisher to make it
        actually useful, but the effect and verbosity can be extrapolated from
        the above.
        
        Most importantly, sending a query request over the wire, you can
        reference the outer scope:
        
        ::
        
           DQ('(p.name, ["(count(b.id)) Book{Publisher.id == b.publisher} b"]) Publisher p')
        
        the subquery output expression references the outer scope. It evaluates
        to the following SQL:
        
        .. code:: sql
        
           SELECT
              "books_publisher"."name",
              (SELECT count("books_book"."id") FROM books_book WHERE "books_publisher"."id" = "books_book"."publisher_id")
           FROM books_publisher
        
        There are some constraints on using subqueries like this. The subquery
        cannot contain any joins.
        
        Sample Project
        --------------
        
        If you want to use Djaq right away in your own test project and you feel
        confident, crack on. In that case skip the following instructions for
        using the sample Bookshop project. Or, if you want to try the sample
        project, clone the django repo:
        
        ::
        
           git clone git@github.com:paul-wolf/djaq.git
           cd djaq/bookshop
        
        If you clone the repo and use the sample project, you don’t need to
        include Djaq as a requirement because it’s included as a module by a
        softlink. Create the virtualenv:
        
        ::
        
           python -m venv .venv
        
        Activate the virtual environment:
        
        ::
        
           source .venv/bin/activate
        
        The module itself does not install Django and there are no further
        requirements. To install dependencies for the sample application:
        
        ::
        
           pip install -r requirements.txt
        
        Now make sure there is a Postgresql instance running. The settings are
        like this:
        
        .. code:: python
        
           DATABASES = {
               'default': {
                   'ENGINE': 'django.db.backends.postgresql_psycopg2',
                   'NAME': 'bookshop',
               },
        
        So, it assumes peer authentication. Change to suite your needs. Now you
        can migrate. Make sure the virtualenv is activated!
        
        ::
        
           ./manage.py migrate
        
        We provide a script to create some sample data:
        
        ::
        
           ./manage.py build_data --book-count 2000
        
        This creates 2000 books and associated data.
        
        The example app comes with a management command to run queries:
        
        ::
        
           ./manage.py djaq "(Publisher.name, max(Book.price) - round(avg(Book.price)) as diff) Book b"  --format json
        
        Output of the command should look like this:
        
        ::
        
           ▶ ./manage.py djaq "(Publisher.name, max(Book.price) - round(avg(Book.price)) as diff) Book b"  --format json
           SELECT books_publisher.name, (max(books_book.price) - round(avg(books_book.price))) FROM books_book LEFT JOIN books_publisher ON (books_book.publisher_id = books_publisher.id)  GROUP BY books_publisher.name LIMIT 10
           {"publisher_name": "Avila, Garza and Ward", "diff": 14.0}
           {"publisher_name": "Boyer-Clements", "diff": 16.0}
           {"publisher_name": "Clark, Garza and York", "diff": 15.0}
           {"publisher_name": "Clarke PLC", "diff": 14.0}
           {"publisher_name": "Griffin-Blake", "diff": 16.0}
           {"publisher_name": "Hampton-Davis", "diff": 13.0}
           {"publisher_name": "Jones LLC", "diff": 15.0}
           {"publisher_name": "Lane-Kim", "diff": 15.0}
           {"publisher_name": "Norris-Bennett", "diff": 14.0}
           {"publisher_name": "Singleton-King", "diff": 17.0}
        
        Notice the SQL used to retrieve data is printed first.
        
        The best approach now would be to trial various queries using the Djaq
        UI as explained above.
        
Platform: UNKNOWN
Classifier: Development Status :: 2 - Pre-Alpha
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: MIT License
Classifier: Programming Language :: Python
Classifier: Programming Language :: Python :: 3.6
Classifier: Programming Language :: Python :: 3.7
Classifier: Programming Language :: Python :: 3.8
Classifier: Framework :: Django
Classifier: Topic :: Software Development :: Libraries :: Python Modules
