Topic guide Models

NOTE: The Models Reference Covers all the model related APIs including model fields, related > objects, and QuerySet.

Topic guide Multiple databases Manually selecting a database

For example, say you have a custom manager method that touches the database – User.objects.create_user(). Because create_user() is a manager method, not a QuerySet method, you can’t do User.objects.using('new_users').create_user().

Topic guide Multiple databases

Django’s admin doesn’t have any explicit support for multiple databases.

Topic guide Database access optimization

…including: Indexes. This is a number one priority, after you have determined from profiling what indexes should be added. Use Meta.indexes or Field.db_index to add these from Django.

Topic guide Database access optimization Retrieve related objects efficiently

Use the FETCH_PEERS fetch mode to make on-demand field access more efficient with bulk-fetching. Enable it for all usage of your models with a custom manager.

Topic guide Database access optimization Don’t retrieve things you don’t need

If a model has a default ordering (Meta.ordering) and you don’t need it, remove it on a QuerySet by calling order_by() with no parameters. Adding an index to your database may help to improve ordering performance.

Topic guide Database access optimization Use bulk methods

When inserting objects into ManyToManyFields, use add() with multiple objects to reduce the number of SQL queries. For example: …is preferable to: …where Band and Artist are models with a many-to-many relationship.

Topic guide Database access optimization Use bulk methods

For example: …is preferable to: …where Band and Artist are models with a many-to-many relationship. When removing multiple many-to-many relationships involving several instances of the related models, use the delete() method on a filtered queryset of the field’s through model.

Topic guide Making queries Asynchronous queries

… you can swap to async for: Be aware that you also can’t do other things that might iterate over the queryset, such as wrapping list() around it to force its evaluation (you can use async for in a comprehension, if …

Topic guide Making queries Querying JSONFieldKey, index, and path transforms

classKT(lookup) : Represents the text value of a key, index, or path transform of JSONField. You can use the double underscore notation in lookup to chain dictionary key and index transforms.

Topic guide Making queries

Example: You can also delete objects in bulk. Every QuerySet has a delete() method, which deletes all members of that QuerySet.

Topic guide Making queries

These accessors may be prefetched by the QuerySet methods select_related() or prefetch_related(). If not prefetched, access will trigger an on-demand fetch through the model’s fetch mode.

Topic guide Making queries Related objectsOne-to-many relationships

This Manager returns QuerySet instances, which can be filtered and manipulated as described in the “Retrieving objects” section above. Example: You can override the FOO_set name by setting the related_name parameter in the ForeignKey definition.

Topic guide Making queries Related objectsOne-to-many relationships

If you would like to specify a different manager for a given query you can use the following syntax: If EntryManager performed default filtering in its get_queryset() method, that filtering would apply to the all() call.

Topic guide

For example: The QuerySet API is extensive. You can annotate and aggregate using many built-in database functions. Beyond those, you can create > custom query expressions. Before using raw SQL, explore the ORM.

Topic guide Performing raw SQL queries

The raw() manager method can be used to perform raw SQL queries that return model instances: Manager.raw(raw_query,params=(),translations=None) This method takes a raw SQL query, executes it, and returns a django.db.models.query.RawQuerySet instance.

Topic guide Creating forms from models Model formsets

As with regular formsets, you can use the max_num and extra parameters to modelformset_factory() to limit the number of extra forms displayed.

Topic guide Django shortcut functions get_list_or_404()

However, you can also pass a QuerySet instance: The above example is a bit contrived since it’s equivalent to doing: but it can be useful if you are passed the queryset variable from somewhere else.

Topic guide Migrations Migration files

… in RunPython operations. This is done by defining a use_in_migrations attribute on the manager class: If you are using the from_queryset() function to dynamically generate a manager class, you need to inherit from the generated class to make it importable: …

Topic guide Performance and optimization General approachesGet things right from the start

For example, you might find that you could calculate the same thing - the number of items in a collection, perhaps - in a QuerySet, in Python, or in a template.