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Diffstat (limited to 'docs/ref/databases.txt')
| -rw-r--r-- | docs/ref/databases.txt | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/docs/ref/databases.txt b/docs/ref/databases.txt index 15dfc40b57..3578a939f6 100644 --- a/docs/ref/databases.txt +++ b/docs/ref/databases.txt @@ -429,6 +429,22 @@ MySQL does not support the ``NOWAIT`` option to the ``SELECT ... FOR UPDATE`` statement. If ``select_for_update()`` is used with ``nowait=True`` then a ``DatabaseError`` will be raised. +Automatic typecasting can cause unexpected results +-------------------------------------------------- + +When performing a query on a string type, but with an integer value, MySQL will +coerce the types of all values in the table to an integer before performing the +comparison. If your table contains the values ``'abc'``, ``'def'`` and you +query for ``WHERE mycolumn=0``, both rows will match. Similarly, ``WHERE mycolumn=1`` +will match the value ``'abc1'``. Therefore, string type fields included in Django +will always cast the value to a string before using it in a query. + +If you implement custom model fields that inherit from :class:`~django.db.models.Field` +directly, are overriding :meth:`~django.db.models.Field.get_prep_value`, or use +:meth:`extra() <django.db.models.query.QuerySet.extra>` or +:meth:`raw() <django.db.models.Manager.raw>`, you should ensure that you +perform the appropriate typecasting. + .. _sqlite-notes: SQLite notes |