mirror of
https://github.com/django/django.git
synced 2024-12-30 13:05:45 +00:00
114 lines
4.2 KiB
Plaintext
114 lines
4.2 KiB
Plaintext
========================
|
|
Database instrumentation
|
|
========================
|
|
|
|
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.monotonic()
|
|
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.monotonic() - 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.
|