#19848: ANDed Q filters spanning generic relations evaluate unexpectedly to
empty
querie sets
-------------------------------------+-------------------------------------
Reporter: gotoalanlu@… | Owner: nobody
Type: Bug | Status: closed
Component: | Version: master
contrib.contenttypes | Resolution: invalid
Severity: Normal | Triage Stage:
Keywords: generic | Unreviewed
genericrelations filter q and | Needs documentation: 0
Has patch: 0 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by Alan Lu <gotoalanlu@…>):
Apologies: my original post contained an error. I meant to refer to the
distributive property of AND and OR. Therefore, in my original line of
thought on the workaround, queries like
{{{
query = (Q(objs__name = "foo") & ~Q(objs__name = "bar")) | Q(objs__name =
"baz")
ExampleModel.filter(query)
}}}
would instead be written as
{{{
query1 = Q(objs__name = "foo") | Q(objs__name = "baz")
query2 = ~Q(objs__name = "bar") | Q(objs__name = "baz")
ExampleModel.filter(query1).filter(query2)
}}}
though DeMorgan's laws could certainly still be useful when unwrapping
deeply nested logical statements with NOTs on parenthetical groups.
The intent of the code is to query for instances of a model with either a
relationship with an object "foo" and no relationship with an object "bar"
or a relationship with an object "baz."
Because I was unaware of the way SQL handles these relationships, I
misjudged the capacity of the system, so it appears that the workaround is
natural. Having read about SQL joins, I am now convinced that Django is
working as designed, even though I am a bit surprised.
--
Ticket URL: <https://code.djangoproject.com/ticket/19848#comment:5>
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