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

Reference Constraints reference UniqueConstraint

UniqueConstraint.condition A Q object that specifies the condition you want the constraint to enforce. For example: ensures that each user only has one draft. These conditions have the same database restrictions as Index.condition.

4.2 5.0

Tutorial Writing your first Django app, part 5 Test a view

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.

Reference Model class reference Attributes

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

Reference Query Expressions Built-in ExpressionsF() expressions

F() is also very useful in QuerySet filters, where they make it possible to filter a set of objects against criteria based on their field values, rather than on Python values. This is documented in using F() expressions in queries.

Topic guide Aggregation Aggregations and other QuerySet clausesfilter() and exclude()

It’s more efficient to use QuerySet.filter() to exclude > rows. The aggregation filter argument is only useful when using two or > more aggregations over the same relations with different conditionals.

Topic guide Aggregation Aggregations and other QuerySet clausesfilter() and exclude()

When developing a complex query that involves both annotate() and filter() clauses, pay particular attention to the order in which the clauses are applied to the QuerySet.

Topic guide Aggregation Aggregations and other QuerySet clauses

For example, to order a QuerySet of books by the number of authors that have contributed to the book, you could use the following query:

Topic guide Aggregation Aggregations and other QuerySet clauses

Ordinarily, annotations are generated on a per-object basis - an annotated QuerySet will return one result for each object in the original QuerySet.

Topic guide Aggregation Aggregations and other QuerySet clausesvalues()

Fields that are mentioned in the order_by() part of a queryset are used when selecting the output data, even if they are not otherwise specified in the values() call.

Topic guide Aggregation Aggregations and other QuerySet clauses

When using the values() clause to group query results for annotations in MySQL with the ONLY_FULL_GROUP_BY SQL mode enabled, you may need to apply AnyValue if the annotation includes a mix of aggregate and non-aggregate expressions.

6.0 dev

Topic guide Database access optimization Understand QuerySets

As well as caching of the whole QuerySet, there is caching of the result of attributes on ORM objects. In general, attributes that are not callable will be cached.

Topic guide Database access optimization Understand QuerySets

To make use of the caching behavior of QuerySet, you may need to use the with template tag.

Topic guide Database access optimization Understand QuerySets

When you have a lot of objects, the caching behavior of the QuerySet can cause a large amount of memory to be used. In this case, iterator() may help.

Topic guide Database access optimization Retrieve related objects efficiently

Where prefetch_related would be useful after the queryset has been evaluated, use prefetch_related_objects() to execute an extra prefetch.

dev

Topic guide Making queries Retrieving objects

filter() will always give you a QuerySet, even if only a single object matches the query - in this case, it will be a QuerySet containing a single element.

Topic guide Making queries Retrieving objectsLookups that span relationships

NOTE: The behavior of filter() for queries > that span multi-value relationships, as described above, is not implemented > equivalently for exclude(). Instead, > the conditions in a single exclude() > call will not necessarily refer to the same item.

Topic guide Making queries Related objectsOne-to-many relationships

In addition to the QuerySet methods defined in “Retrieving objects” above, the ForeignKey Manager has additional methods used to handle the set of related objects.

Reference PostgreSQL specific model fields Range FieldsQuerying Range Fields

Range fields support the standard lookups: lt, gt, lte and gte. These are not particularly helpful - they compare the lower bounds first and then the upper bounds only if necessary.

Topic guide Aggregation Generating aggregates for each item in a QuerySet

… most aggregates, there is no way to avoid this problem, however, the Count aggregate has a distinct parameter that may help: NOTE: In order to understand what happens in your query, consider inspecting the > query property of your QuerySet.