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Add a safety note on using the new delete rules.

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Vincent Driessen 2010-12-21 01:19:27 -08:00
parent 0f68df3b4a
commit 03a757bc6e
1 changed files with 23 additions and 0 deletions
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docs/guide/defining-documents.rst
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@ -193,6 +193,7 @@ as the constructor's argument::
class ProfilePage(Document): class ProfilePage(Document):
content = StringField() content = StringField()
Dealing with deletion of referred documents Dealing with deletion of referred documents
''''''''''''''''''''''''''''''''''''''''''' '''''''''''''''''''''''''''''''''''''''''''
By default, MongoDB doesn't check the integrity of your data, so deleting By default, MongoDB doesn't check the integrity of your data, so deleting
@ -207,6 +208,11 @@ supplying the :attr:`reverse_delete_rule` attributes on the
... ...
profile_page = ReferenceField('ProfilePage', reverse_delete_rule=mongoengine.NULLIFY) profile_page = ReferenceField('ProfilePage', reverse_delete_rule=mongoengine.NULLIFY)
The declaration in this example means that when an :class:`Employee` object is
removed, the :class:`ProfilePage` that belongs to that employee is removed as
well. If a whole batch of employees is removed, all profile pages that are
linked are removed as well.
Its value can take any of the following constants: Its value can take any of the following constants:
:const:`mongoengine.DO_NOTHING` :const:`mongoengine.DO_NOTHING`
@ -223,6 +229,23 @@ Its value can take any of the following constants:
are deleted first. are deleted first.
.. warning::
A safety note on setting up these delete rules! Since the delete rules are
not recorded on the database level by MongoDB itself, but instead at runtime,
in-memory, by the MongoEngine module, it is of the upmost importance
that the module that declares the relationship is loaded **BEFORE** the
delete is invoked.
If, for example, the :class:`Employee` object lives in the
:mod:`payroll` app, and the :class:`ProfilePage` in the :mod:`people`
app, it is extremely important that the :mod:`people` app is loaded
before any employee is removed, because otherwise, MongoEngine could
never know this relationship exists.
In Django, be sure to put all apps that have such delete rule declarations in
their :file:`models.py` in the :const:`INSTALLED_APPS` tuple.
Generic reference fields Generic reference fields
'''''''''''''''''''''''' ''''''''''''''''''''''''
A second kind of reference field also exists, A second kind of reference field also exists,