Reference Query Expressions Built-in ExpressionsSubquery() expressions

classExists(queryset)[source] Exists is a Subquery subclass that uses an SQL EXISTS statement. In many cases it will perform better than a subquery since the database is able to stop evaluation of the subquery when a first matching row is found.

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Topic guide Database access optimization Understand QuerySets

QuerySet.explain() gives you detailed information about how the database executes a query, including indexes and joins that are used.

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Reference The contenttypes framework

classGenericPrefetch(lookup,querysets,to_attr=None)[source] This lookup is similar to Prefetch() and it should only be used on GenericForeignKey. The querysets argument accepts a list of querysets, each for a different ContentType. This is useful for GenericForeignKey with non-homogeneous set of results.

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Reference PostgreSQL specific model fields

The contains lookup is overridden on ArrayField. The returned objects will be those where the values passed are a subset of the data. It uses the SQL operator @>. For example:

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Reference The contenttypes framework Generic relations

classGenericPrefetch(lookup,querysets,to_attr=None)[source] This lookup is similar to Prefetch() and it should only be used on GenericForeignKey. The querysets argument accepts a list of querysets, each for a different ContentType. This is useful for GenericForeignKey with non-homogeneous set of results.

5.0 5.1 5.2 6.0

Reference PostgreSQL specific model fields Range FieldsQuerying Range FieldsContainment functions

The contains lookup is overridden on ArrayField. The returned objects will be those where the values passed are a subset of the data. It uses the SQL operator @>. For example:

4.2 5.0 5.1 5.2 6.0

Reference Databases

When using QuerySet.iterator(), Django opens a server-side cursor. By default, PostgreSQL assumes that only the first 10% of the results of cursor queries will be fetched.

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Topic guide Making queries

The QuerySet returned by all() describes all objects in the database table. Usually, though, you’ll need to select only a subset of the complete set of objects. To create such a subset, you refine the initial QuerySet, adding filter conditions.

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Topic guide Making queries

The result of refining a QuerySet is itself a QuerySet, so it’s possible to chain refinements together. For example: This takes the initial QuerySet of all entries in the database, adds a filter, then an exclusion, then another filter.

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Reference Databases PostgreSQL notes

When using QuerySet.iterator(), Django opens a server-side cursor. By default, PostgreSQL assumes that only the first 10% of the results of cursor queries will be fetched.

4.2 5.0 5.1 5.2 6.0

Topic guide Making queries Retrieving objects

The QuerySet returned by all() describes all objects in the database table. Usually, though, you’ll need to select only a subset of the complete set of objects. To create such a subset, you refine the initial QuerySet, adding filter conditions.

4.2 5.0 5.1 5.2 6.0

Topic guide Making queries Retrieving objectsRetrieving specific objects with filters

The result of refining a QuerySet is itself a QuerySet, so it’s possible to chain refinements together. For example: This takes the initial QuerySet of all entries in the database, adds a filter, then an exclusion, then another filter.

4.2 5.0 5.1 5.2 6.0

Reference PostgreSQL specific database constraints ExclusionConstraint

ExclusionConstraint.condition A Q object that specifies the condition to restrict a constraint to a subset of rows. For example, condition=Q(cancelled=False). These conditions have the same database restrictions as django.db.models.Index.condition.

Reference Constraints reference

CheckConstraint.condition A Q object or boolean Expression that specifies the conditional check you want the constraint to enforce. For example: ensures the age field is never less than 18.

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Topic guide Making queries

The contains lookup is overridden on JSONField. The returned objects are those where the given dict of key-value pairs are all contained in the top-level of the field. For example: NOTE: contains is not supported on Oracle and SQLite.

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Reference Constraints reference UniqueConstraint

CheckConstraint.condition A Q object or boolean Expression that specifies the conditional check you want the constraint to enforce. For example: ensures the age field is never less than 18.

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Topic guide Making queries Querying JSONFieldContainment and key lookups

The contains lookup is overridden on JSONField. The returned objects are those where the given dict of key-value pairs are all contained in the top-level of the field. For example: NOTE: contains is not supported on Oracle and SQLite.

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Reference Constraints reference CheckConstraint

CheckConstraint.check A Q object or boolean Expression that specifies the check you want the constraint to enforce. For example, CheckConstraint(check=Q(age__gte=18), name='age_gte_18') ensures the age field is never less than 18.

4.2 5.0

Tutorial Writing your first Django app, part 5

We ought to add a similar get_queryset method to ResultsView and create a new test class for that view. It’ll be very similar to what we have just created; in fact there will be a lot of repetition.

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Reference Model class reference

exceptionModel.MultipleObjectsReturned : This exception is raised by QuerySet.get() when multiple objects are found for the given lookups.

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