add limitations

Author: jordan-smith721

source/index.txt

@@ -11,6 +11,7 @@ Django MongoDB Backend
 
 .. toctree::
 
+    Limitations </limitations>
     Issues & Help </issues-and-help>
     Compatibility </compatibility>
 

source/limitations.txt

@@ -0,0 +1,197 @@
+.. _django-limitations:
+
+============
+Limitations
+============
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 1
+   :class: singlecol
+
+.. facet::
+   :name: genre
+   :values: reference
+
+.. meta::
+   :keywords: support, features, django
+
+Overview
+--------
+
+On this page, you can find a list of features that
+{+django-odm+} does not support. Because {+django-odm+} is in active
+development, some features listed on this page might be considered for future
+releases based on customer demand.
+
+Unsupported Database Variables
+------------------------------
+
+The following database variables are not supported by {+django-odm+}:
+
+- ``ATOMIC_REQUESTS``
+- ``AUTOCOMMIT``
+- ``CONN_HEALTH_CHECKS``
+- ``TIME_ZONE``
+
+Model Limitations
+-----------------
+
+The following model limitations apply to {+django-odm+}:
+
+- {+django-odm+} does not support multiple models within a collection.
+
+Indexes
+~~~~~~~
+
+{+django-odm+} does not support the following index functionalities:
+
+- Creation of ``$vectorSearch`` and ``$search`` indexes through the Django
+  Indexes API
+- Creating geospatial indexes through the Django Indexes API
+- Updating indexes in ``EmbeddedModelFields`` after model creation
+
+Fields
+~~~~~~
+
+{+django-odm+} has the following limitations on the specified field types:
+
+- ``ArrayField``
+
+  - {+django-odm+} does not support ``ArrayField`` polymorphism.
+  - {+django-odm+} does not support ``EmbeddedModelField`` within an ``ArrayField``.
+
+- ``EmbeddedModelField``
+
+  - You cannot change embedded models after creation.
+  - Embedded documents cannot take Django foreign keys.
+  - {+django-odm+} does not support arbitrary or untyped embedded model
+    fields. You must derive all fields from a ``Model`` class.
+
+- ``JSONField``
+
+  - {+django-odm+} cannot distinguish between a JSON and a SQL ``null`` value.
+    Queries that use ``Value(None, JSONField())`` or the ``isnull`` lookup
+    return both JSON and SQL ``null`` values.
+  - Some queries with ``Q`` objects, such as ``Q(value__foo="bar")``, might
+    not work as expected.
+  - Filtering with a ``None`` key incorrectly returns objects in which a key
+    does not exist.
+
+- ``DateTimeField``
+
+  - {+django-odm+} does not support microsecond granularity for
+  ``DateTimeField``.
+
+- ``DurationField``
+
+  - The ``DurationField`` stores milliseconds rather than microseconds.
+
+The following field types are unavailable in {+django-odm+}:
+
+- ``GeneratedField``
+- ``ImageField``
+
+- ``ForeignKey``
+
+  - When possible, you should use an ``EmbeddedModelField`` instead of a
+    ``ForeignKey`` field to avoid using ``$lookup`` operations. An
+    ``EmbeddedModelField`` emulates a MongoDB embedded document, and performs
+    better than a ``ForeignKey`` field. To learn more about how to reduce
+    ``$lookup`` operations, see the :atlas:`Reduce $lookup Operations
+    </schema-suggestions/reduce-lookup-operations/>` guide in the Atlas
+    documentation.
+  - The performance of `CASCADE deletes <{+django-docs+}/ref/models/fields/#foreignkey>`__ 
+    on a ``ForeignKey`` field is not as performant as using an
+    ``EmbeddedModelField``.
+
+Querying Limitations
+--------------------
+
+{+django-odm+} does not support the following ``QuerySet`` API methods:
+
+- ``distinct()``
+- ``dates()``
+- ``datetimes()``
+- ``prefetch_related()``
+- ``extra()``
+
+{+django-odm+} does not support ``QuerySet.delete()`` and ``update()`` queries
+that span multiple collections.
+
+Geospatial Queries
+~~~~~~~~~~~~~~~~~~
+
+- {+django-odm+} does not support ``GeoDjango``.
+- {+django-odm+} does not have any ``$lookup`` operators for MongoDB-specific
+  geospatial queries.
+
+Aggregation operators
+~~~~~~~~~~~~~~~~~~~~~
+
+{+django-odm+} does not contain any custom Django Field lookups for the MongoDB
+aggregation framework. Instead, use the ``raw_aggregate`` method. For more
+information on the ``raw_aggregate`` method. 
+
+.. TODO: Link to aggregation
+
+Database Functions
+~~~~~~~~~~~~~~~~~~
+
+{+django-odm+} does not support the following database functions:
+
+- ``Chr``
+- ``ExtractQuarter``
+- ``MD5``
+- ``Now``
+- ``Ord``
+- ``Pad``
+- ``Repeat``
+- ``Reverse``
+- ``Right``
+- ``SHA1``, ``SHA224``, ``SHA256``, ``SHA384``, ``SHA512``
+- ``Sign``
+
+The ``tzinfo`` parameter of the ``Trunc`` database functions doesn't work
+properly because MongoDB converts the result back to UTC.
+
+Django Management Command Limitations
+-------------------------------------
+
+{+django-odm+} does not support the following Django management commands:
+
+- ``createcachetable``
+- ``inspectdb``
+- ``optimizemigration``
+- ``sqlflush``
+- ``sqlsequencereset``
+
+Migration Limitations
+---------------------
+
+- {+django-odm+} does not support enforced schema validation. To learn how to
+  enforce schema validation in your application, see the :manual:`Specify JSON
+  Schema Validation </core/schema-validation/specify-json-schema/>` guide in the
+  {+mdb-server+} manual.
+- {+django-odm+} does not support `DDL Transactions
+  <{+django-docs+}/topics/migrations/#transactions>`__.
+- {+django-odm+} does not support the ``migrate --fake-initial`` command.
+
+Asynchronous Support
+--------------------
+
+{+django-odm+} has not been tested for support of the asynchronous functionality of
+the Django API.
+
+Data Types
+----------
+
+{+django-odm+} does not have a custom ``Field`` class for the ``BSONRegExp``
+data type. Instead, use the ``CharField`` class.
+
+Performance
+-----------
+
+The engineering team is prioritizing performance improvements prior to the
+General Availability release of {+django-odm+}.