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Mike Bayer
59 lines
2.5 KiB
ReStructuredText
59 lines
2.5 KiB
ReStructuredText
.. module:: sqlalchemy.orm
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.. _mapping_constructors:
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Constructors and Object Initialization
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=======================================
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Mapping imposes no restrictions or requirements on the constructor
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(``__init__``) method for the class. You are free to require any arguments for
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the function that you wish, assign attributes to the instance that are unknown
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to the ORM, and generally do anything else you would normally do when writing
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a constructor for a Python class.
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The SQLAlchemy ORM does not call ``__init__`` when recreating objects from
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database rows. The ORM's process is somewhat akin to the Python standard
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library's ``pickle`` module, invoking the low level ``__new__`` method and
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then quietly restoring attributes directly on the instance rather than calling
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``__init__``.
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If you need to do some setup on database-loaded instances before they're ready
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to use, you can use the ``@reconstructor`` decorator to tag a method as the
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ORM counterpart to ``__init__``. SQLAlchemy will call this method with no
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arguments every time it loads or reconstructs one of your instances. This is
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useful for recreating transient properties that are normally assigned in your
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``__init__``::
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from sqlalchemy import orm
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class MyMappedClass(object):
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def __init__(self, data):
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self.data = data
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# we need stuff on all instances, but not in the database.
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self.stuff = []
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@orm.reconstructor
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def init_on_load(self):
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self.stuff = []
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When ``obj = MyMappedClass()`` is executed, Python calls the ``__init__``
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method as normal and the ``data`` argument is required. When instances are
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loaded during a :class:`~sqlalchemy.orm.query.Query` operation as in
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``query(MyMappedClass).one()``, ``init_on_load`` is called.
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Any method may be tagged as the :func:`~sqlalchemy.orm.reconstructor`, even
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the ``__init__`` method. SQLAlchemy will call the reconstructor method with no
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arguments. Scalar (non-collection) database-mapped attributes of the instance
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will be available for use within the function. Eagerly-loaded collections are
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generally not yet available and will usually only contain the first element.
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ORM state changes made to objects at this stage will not be recorded for the
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next flush() operation, so the activity within a reconstructor should be
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conservative.
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:func:`~sqlalchemy.orm.reconstructor` is a shortcut into a larger system
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of "instance level" events, which can be subscribed to using the
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event API - see :class:`.InstanceEvents` for the full API description
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of these events.
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.. autofunction:: reconstructor
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