Commit 8ec388a6 authored by Tim Graham's avatar Tim Graham
Browse files

Fixed #22442 -- Provided additional documentation regarding id fields clashing. 

Thanks benjaoming for raising the issue and Loic for the examples.
parent d9c272b2

docs/releases/1.7.txt

+1 −0
Original line number Diff line number Diff line
@@ -1194,6 +1194,7 @@ Miscellaneous 
  fields in the model inheritance hierarchy results in a system check error.

  For example, if you use multi-inheritance, you need to define custom primary

  key fields on parent models, otherwise the default ``id`` fields will clash.

  See :ref:`model-multiple-inheritance-topic` for details.



* ``django.utils.translation.parse_accept_lang_header()`` now returns

  lowercase locales, instead of the case as it was provided. As locales should

docs/topics/db/models.txt

+63 −0
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@@ -1262,6 +1262,8 @@ So, the general rules are: 
   This sets things up so that the proxy model is an exact copy of the

   storage structure of the original model when data is saved.



.. _model-multiple-inheritance-topic:



Multiple inheritance

--------------------
@@ -1279,6 +1281,67 @@ inheritance hierarchies as simple and straightforward as possible so that you 
won't have to struggle to work out where a particular piece of information is

coming from.



.. versionchanged:: 1.7



Before Django 1.7, inheriting from multiple models that had an ``id`` primary

key field did not raise an error, but could result in data loss. For example,

consider these models (which no longer validate due to the clashing ``id``

fields)::



    class Article(models.Model):

        headline = models.CharField(max_length=50)

        body = models.TextField()



    class Book(models.Model):

        title = models.CharField(max_length=50)



    class BookReview(Book, Article):

        pass



This snippet demonstrates how creating a child object overwrote the value of a

previously created parent object::



    >>> article = Article.objects.create(headline='Some piece of news.')

    >>> review = BookReview.objects.create(

    ...     headline='Review of Little Red Riding Hood.',

    ...     title='Little Red Riding Hood')

    >>>

    >>> assert Article.objects.get(pk=article.pk).headline == article.headline

    Traceback (most recent call last):

      File "<console>", line 1, in <module>

    AssertionError

    >>> # the "Some piece of news." headline has been overwritten.

    >>> Article.objects.get(pk=article.pk).headline

    'Review of Little Red Riding Hood.'



To properly use multiple inheritance, you can use an explicit

:class:`~django.db.models.AutoField` in the base models::



    class Article(models.Model):

        article_id = models.AutoField(primary_key=True)

        ...



    class Book(models.Model):

        book_id = models.AutoField(primary_key=True)

        ...



    class BookReview(Book, Article):

        pass



Or use a common ancestor to hold the :class:`~django.db.models.AutoField`::



    class Piece(models.Model):

        pass



    class Article(Piece):

        ...



    class Book(Piece):

        ...



    class BookReview(Book, Article):

        pass



Field name "hiding" is not permitted

-------------------------------------