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Database instrumentation
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To help you understand and control the queries issued by your code, Django
provides a hook for installing wrapper functions around the execution of
database queries. For example, wrappers can count queries, measure query
duration, log queries, or even prevent query execution (e.g. to make sure that
no queries are issued while rendering a template with prefetched data).

The wrappers are modeled after :doc:`middleware </topics/http/middleware>` --
they are callables which take another callable as one of their arguments. They
call that callable to invoke the (possibly wrapped) database query, and they
can do what they want around that call. They are, however, created and
installed by user code, and so don't need a separate factory like middleware do.

Installing a wrapper is done in a context manager -- so the wrappers are
temporary and specific to some flow in your code.

As mentioned above, an example of a wrapper is a query execution blocker. It
could look like this::

    def blocker(*args):
        raise Exception('No database access allowed here.')

And it would be used in a view to block queries from the template like so::

    from django.db import connection
    from django.shortcuts import render

    def my_view(request):
        context = {...}  # Code to generate context with all data.
        template_name = ...
        with connection.execute_wrapper(blocker):
            return render(request, template_name, context)

The parameters sent to the wrappers are:

* ``execute`` -- a callable, which should be invoked with the rest of the
  parameters in order to execute the query.

* ``sql`` -- a ``str``, the SQL query to be sent to the database.

* ``params`` -- a list/tuple of parameter values for the SQL command, or a
  list/tuple of lists/tuples if the wrapped call is ``executemany()``.

* ``many`` -- a ``bool`` indicating whether the ultimately invoked call is
  ``execute()`` or ``executemany()`` (and whether ``params`` is expected to be
  a sequence of values, or a sequence of sequences of values).

* ``context`` -- a dictionary with further data about the context of
  invocation. This includes the connection and cursor.

Using the parameters, a slightly more complex version of the blocker could
include the connection name in the error message::

    def blocker(execute, sql, params, many, context):
        alias = context['connection'].alias
        raise Exception("Access to database '{}' blocked here".format(alias))

For a more complete example, a query logger could look like this::

    import time

    class QueryLogger:

        def __init__(self):
            self.queries = []

        def __call__(self, execute, sql, params, many, context):
            current_query = {'sql': sql, 'params': params, 'many': many}
            start = time.time()
            try:
                result = execute(sql, params, many, context)
            except Exception as e:
                current_query['status'] = 'error'
                current_query['exception'] = e
                raise
            else:
                current_query['status'] = 'ok'
                return result
            finally:
                duration = time.time() - start
                current_query['duration'] = duration
                self.queries.append(current_query)

To use this, you would create a logger object and install it as a wrapper::

    from django.db import connection

    ql = QueryLogger()
    with connection.execute_wrapper(ql):
        do_queries()
    # Now we can print the log.
    print(ql.queries)

.. currentmodule:: django.db.backends.base.DatabaseWrapper

``connection.execute_wrapper()``
--------------------------------

.. method:: execute_wrapper(wrapper)

Returns a context manager which, when entered, installs a wrapper around
database query executions, and when exited, removes the wrapper. The wrapper is
installed on the thread-local connection object.

``wrapper`` is a callable taking five arguments.  It is called for every query
execution in the scope of the context manager, with arguments ``execute``,
``sql``, ``params``, ``many``, and ``context`` as described above. It's
expected to call ``execute(sql, params, many, context)`` and return the return
value of that call.
