============================
Known issues and limitations
============================

This document summarizes some known issues and limitations of this library.
If you notice an issue not listed, use the :ref:`issue-tracker` to report a bug
or request a feature.

Like any database, MongoDB has some particularities. Also keep in mind that
because MongoDB is a NoSQL database, it's impossible to implement SQL-specific
functionality.

Model fields
============

- :class:`~django.db.models.DateTimeField` is limited to millisecond precision
  (rather than microsecond like most other databases), and correspondingly,
  :class:`~django.db.models.DurationField` stores milliseconds rather than
  microseconds.

- Some of Django's built-in fields aren't supported by MongoDB:

  - :class:`~django.db.models.AutoField` (including
    :class:`~django.db.models.BigAutoField` and
    :class:`~django.db.models.SmallAutoField`)
  - :class:`~django.db.models.CompositePrimaryKey`
  - :class:`~django.db.models.GeneratedField`

.. _known-issues-querying:

Querying
========

- The following ``QuerySet`` methods aren't supported:

  - :meth:`~django.db.models.query.QuerySet.extra`
  - :meth:`~django.db.models.query.QuerySet.prefetch_related`
  - :meth:`~django.db.models.query.QuerySet.raw` (use
    :meth:`~django_mongodb_backend.queryset.MongoQuerySet.raw_aggregate`
    instead)
  - :meth:`~django.db.models.query.QuerySet.select_for_update` (acts as a
    no-op)

    .. versionadded:: 6.0.2

        Support for :meth:`~django.db.models.query.QuerySet.difference` and
        :meth:`~django.db.models.query.QuerySet.intersection` was added.

- :meth:`QuerySet.delete() <django.db.models.query.QuerySet.delete>` and
  :meth:`~django.db.models.query.QuerySet.update` do not support queries that
  span multiple collections.

- The :class:`~django.db.models.StringAgg` aggregation function isn't
  supported.

- When querying :class:`~django.db.models.JSONField`:

  - There is no way to distinguish between a JSON ``"null"`` (represented by
    ``Value(None, JSONField())``) and a SQL ``null`` (queried using the
    :lookup:`isnull` lookup). Both of these queries return both of these nulls.
  - Using JSON keys in ``QuerySet.exclude()`` incorrectly treats objects with
    absent keys as satisfying the exclusion.

Database functions
==================

- Some of Django's built-in database functions aren't supported by MongoDB:

  - :class:`~django.db.models.functions.Chr`
  - :class:`~django.db.models.functions.Ord`
  - :class:`~django.db.models.functions.SHA1`,
    :class:`~django.db.models.functions.SHA224`,
    :class:`~django.db.models.functions.SHA384`,
    :class:`~django.db.models.functions.SHA512`

Transaction management
======================

By default, query execution uses Django and MongoDB's default behavior of
autocommit mode. Each query is immediately committed to the database.

Django's :doc:`transaction management APIs <django:topics/db/transactions>`
are not supported. Instead, this package provides its own :doc:`transaction
APIs </topics/transactions>`.

Database introspection
======================

Due to the lack of ability to introspect MongoDB collection schema,
:djadmin:`inspectdb` and :option:`migrate --fake-initial` aren't supported.

Caching
=======

:ref:`Database caching <database-caching>` is not supported.