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[1.8.x] Fixed #24486 -- Documented method to provide output_field to mixed F expressions

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Backport of 820381d38b from master
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Josh Smeaton 2015-03-16 16:11:03 +11:00 • committed by Tim Graham
parent 63b998a653
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docs/ref/models/expressions.txt
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@ -278,6 +278,17 @@ should define the desired ``output_field``. For example, adding an
``IntegerField()`` and a ``FloatField()`` together should probably have
``output_field=FloatField()`` defined.
.. note::
When you need to define the ``output_field`` for ``F`` expression
arithmetic between different types, it's necessary to surround the
expression in another expression::
from django.db.models import DateTimeField, ExpressionNode, F
Race.objects.annotate(finish=ExpressionNode(
F('start') + F('duration'), output_field=DateTimeField()))
.. versionchanged:: 1.8
``output_field`` is a new parameter.