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52 lines
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52 lines
1.7 KiB
Plaintext
=====================
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Query-related classes
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=====================
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.. currentmodule:: django.db.models
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This document provides reference material for query-related tools not
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documented elsewhere.
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``Q()`` objects
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===============
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.. class:: Q
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A ``Q()`` object, like an :class:`~django.db.models.F` object, encapsulates a
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SQL expression in a Python object that can be used in database-related
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operations.
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In general, ``Q() objects`` make it possible to define and reuse conditions.
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This permits the :ref:`construction of complex database queries
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<complex-lookups-with-q>` using ``|`` (``OR``) and ``&`` (``AND``) operators;
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in particular, it is not otherwise possible to use ``OR`` in ``QuerySets``.
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``Prefetch()`` objects
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======================
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.. versionadded:: 1.7
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.. class:: Prefetch(lookup, queryset=None, to_attr=None)
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The ``Prefetch()`` object can be used to control the operation of
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:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
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The ``lookup`` argument describes the relations to follow and works the same
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as the string based lookups passed to
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:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
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The ``queryset`` argument supplies a base ``QuerySet`` for the given lookup.
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This is useful to further filter down the prefetch operation, or to call
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:meth:`~django.db.models.query.QuerySet.select_related()` from the prefetched
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relation, hence reducing the number of queries even further.
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The ``to_attr`` argument sets the result of the prefetch operation to a custom
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attribute.
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.. note::
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When using ``to_attr`` the prefetched result is stored in a list.
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This can provide a significant speed improvement over traditional
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``prefetch_related`` calls which store the cached result within a
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``QuerySet`` instance.
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