known-issues.rst

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.GeneratedField`

Querying

• The following QuerySet methods aren't supported:
  • :meth:`bulk_update() <django.db.models.query.QuerySet.bulk_update>`
  • :meth:`extra() <django.db.models.query.QuerySet.extra>`
  • :meth:`prefetch_related() <django.db.models.query.QuerySet.prefetch_related>`
• :meth:`QuerySet.datetimes() <django.db.models.query.QuerySet.datetimes>` may generate incorrect results. XXX TODO XXX
• :meth:`QuerySet.delete() <django.db.models.query.QuerySet.delete>` and :meth:`update() <django.db.models.query.QuerySet.update>` do not support queries that span multiple collections.
• 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.
  • Some queries with Q objects, e.g. Q(value__foo="bar"), don't work properly, particularly with QuerySet.exclude().
  • Filtering for a None key, e.g. QuerySet.filter(value__j=None) incorrectly returns objects where the key doesn't exist.
  • You can study the skipped tests in DatabaseFeatures.django_test_skips for more details on known issues.

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.ExtractQuarter`
  • :class:`~django.db.models.functions.LPad`, :class:`~django.db.models.functions.RPad`
  • :class:`~django.db.models.functions.MD5`
  • :class:`~django.db.models.functions.Now`
  • :class:`~django.db.models.functions.Ord`
  • :class:`~django.db.models.functions.Repeat`
  • :class:`~django.db.models.functions.Reverse`
  • :class:`~django.db.models.functions.Right`
  • :class:`~django.db.models.functions.SHA1`, :class:`~django.db.models.functions.SHA224`, :class:`~django.db.models.functions.SHA256`, :class:`~django.db.models.functions.SHA384`, :class:`~django.db.models.functions.SHA512`
  • :class:`~django.db.models.functions.Sign`
• The tzinfo parameter of the :class:`~django.db.models.functions.Trunc` database functions doesn't work properly because MongoDB converts the result back to UTC.

Transaction management

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.

Database introspection

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

Caching

:doc:`Database caching </topics/cache>` uses this library's :djadmin:`createcachecollection` command rather Django's SQL-specific :djadmin:`createcachetable`.

Secondly, you must use the :class:`django_mongodb_backend.cache.MongoDBCache` backend rather than Django's built-in database cache backend, django.core.cache.backends.db.DatabaseCache).