django4.1中文手册

========================== ``QuerySet`` API reference ========================== .. currentmodule:: django.db.models.query This document describes the details of the ``QuerySet`` API. It builds on the material presented in the :doc:`model </topics/db/models>` and :doc:`database query </topics/db/queries>` guides, so you'll probably want to read and understand those documents before reading this one. Throughout this reference we'll use the :ref:`example blog models <queryset-model-example>` presented in the :doc:`database query guide </topics/db/queries>`. .. _when-querysets-are-evaluated: When ``QuerySet``\s are evaluated ================================= Internally, a ``QuerySet`` can be constructed, filtered, sliced, and generally passed around without actually hitting the database. No database activity actually occurs until you do something to evaluate the queryset. You can evaluate a ``QuerySet`` in the following ways: * **Iteration.** A ``QuerySet`` is iterable, and it executes its database query the first time you iterate over it. For example, this will print the headline of all entries in the database:: for e in Entry.objects.all(): print(e.headline) Note: Don't use this if all you want to do is determine if at least one result exists. It's more efficient to use :meth:`~QuerySet.exists`. * **Asynchronous iteration.**. A ``QuerySet`` can also be iterated over using ``async for``:: async for e in Entry.objects.all(): results.append(e) Both synchronous and asynchronous iterators of QuerySets share the same underlying cache. .. versionchanged:: 4.1 Support for asynchronous iteration was added. * **Slicing.** As explained in :ref:`limiting-querysets`, a ``QuerySet`` can be sliced, using Python's array-slicing syntax. Slicing an unevaluated ``QuerySet`` usually returns another unevaluated ``QuerySet``, but Django will execute the database query if you use the "step" parameter of slice syntax, and will return a list. Slicing a ``QuerySet`` that has been evaluated also returns a list. Also note that even though slicing an unevaluated ``QuerySet`` returns another unevaluated ``QuerySet``, modifying it further (e.g., adding more filters, or modifying ordering) is not allowed, since that does not translate well into SQL and it would not have a clear meaning either. * **Pickling/Caching.** See the following section for details of what is involved when `pickling QuerySets`_. The important thing for the purposes of this section is that the results are read from the database. * **repr().** A ``QuerySet`` is evaluated when you call ``repr()`` on it. This is for convenience in the Python interactive interpreter, so you can immediately see your results when using the API interactively. * **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it. This, as you might expect, returns the length of the result list. Note: If you only need to determine the number of records in the set (and don't need the actual objects), it's much more efficient to handle a count at the database level using SQL's ``SELECT COUNT(*)``. Django provides a :meth:`~QuerySet.count` method for precisely this reason. * **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on it. For example:: entry_list = list(Entry.objects.all()) * **bool().** Testing a ``QuerySet`` in a boolean context, such as using ``bool()``, ``or``, ``and`` or an ``if`` statement, will cause the query to be executed. If there is at least one result, the ``QuerySet`` is ``True``, otherwise ``False``. For example:: if Entry.objects.filter(headline="Test"): print("There is at least one Entry with the headline Test") Note: If you only want to determine if at least one result exists (and don't need the actual objects), it's more efficient to use :meth:`~QuerySet.exists`. .. _pickling QuerySets: Pickling ``QuerySet``\s ----------------------- If you :mod:`pickle` a ``QuerySet``, this will force all the results to be loaded into memory prior to pickling. Pickling is usually used as a precursor to caching and when the cached queryset is reloaded, you want the results to already be present and ready for use (reading from the database can take some time, defeating the purpose of caching). This means that when you unpickle a ``QuerySet``, it contains the results at the moment it was pickled, rather than the results that are currently in the database. If you only want to pickle the necessary information to recreate the ``QuerySet`` from the database at a later time, pickle the ``query`` attribute of the ``QuerySet``. You can then recreate the original ``QuerySet`` (without any results loaded) using some code like this:: >>> import pickle >>> query = pickle.loads(s) # Assuming 's' is the pickled string. >>> qs = MyModel.objects.all() >>> qs.query = query # Restore the original 'query'. The ``query`` attribute is an opaque object. It represents the internals of the query construction and is not part of the public API. However, it is safe (and fully supported) to pickle and unpickle the attribute's contents as described here. .. admonition:: Restrictions on ``QuerySet.values_list()`` If you recreate :meth:`QuerySet.values_list` using the pickled ``query`` attribute, it will be converted to :meth:`QuerySet.values`:: >>> import pickle >>> qs = Blog.objects.values_list('id', 'name') >>> qs <QuerySet [(1, 'Beatles Blog')]> >>> reloaded_qs = Blog.objects.all() >>> reloaded_qs.query = pickle.loads(pickle.dumps(qs.query)) >>> reloaded_qs <QuerySet [{'id': 1, 'name': 'Beatles Blog'}]> .. admonition:: You can't share pickles between versions Pickles of ``QuerySets`` are only valid for the version of Django that was used to generate them. If you generate a pickle using Django version N, there is no guarantee that pickle will be readable with Django version N+1. Pickles should not be used as part of a long-term archival strategy. Since pickle compatibility errors can be difficult to diagnose, such as silently corrupted objects, a ``RuntimeWarning`` is raised when you try to unpickle a queryset in a Django version that is different than the one in which it was pickled. .. _queryset-api: ``QuerySet`` API ================ Here's the formal declaration of a ``QuerySet``: .. class:: QuerySet(model=None, query=None, using=None, hints=None) Usually when you'll interact with a ``QuerySet`` you'll use it by :ref:`chaining filters <chaining-filters>`. To make this work, most ``QuerySet`` methods return new querysets. These methods are covered in detail later in this section. The ``QuerySet`` class has the following public attributes you can use for introspection: .. attribute:: ordered ``True`` if the ``QuerySet`` is ordered — i.e. has an :meth:`order_by()` clause or a default ordering on the model. ``False`` otherwise. .. attribute:: db The database that will be used if this query is executed now. .. note:: The ``query`` parameter to :class:`QuerySet` exists so that specialized query subclasses can reconstruct internal query state. The value of the parameter is an opaque representation of that query state and is not part of a public API. .. currentmodule:: django.db.models.query.QuerySet Methods that return new ``QuerySet``\s -------------------------------------- Django provides a range of ``QuerySet`` refinement methods that modify either the types of results returned by the ``QuerySet`` or the way its SQL query is executed. .. note:: These methods do not run database queries, therefore they are **safe to** **run in asynchronous code**, and do not have separate asynchronous versions. ``filter()`` ~~~~~~~~~~