#27021: Add explicit support for Q object annotations
-------------------------------------+-------------------------------------
Reporter: jarshwah | Owner: nobody
Type: New | Status: new
feature |
Component: Database | Version: master
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: Accepted | Has patch: 0
Needs documentation: 1 | Needs tests: 1
Patch needs improvement: 1 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
A common pattern that has arisen with expressions is to annotate a Q
object to the query without going through the Case/When ceremony.
{{{
Model.objects.annotate(boolfield=ExpressionWrapper(Q(field__gte=4),
output_field=BooleanField()))
}}}
We've tried to discourage this pattern because the Case/When structure is
a lot more powerful and correct. Sometimes the power isn't required and
users really just want to annotate a boolean expression. Q objects and the
WhereNode it resolves to aren't really proper expressions, but they could
be. Case/When uses Q objects internally, which is why it's important to
test Case/When for any major changes to Q and WhereNode.
Explicit support for Q object annotations should implement an implicit
BooleanField as the output_field. I believe this is only required on the
WhereNode since that is what is ultimately resolved. Once this happens,
the above query becomes a lot nicer.
{{{
Model.objects.annotate(boolfield=Q(field__gte=4))
}}}
For this to work the following is definitely required:
- Add an `output_field=BooleanField()` to `WhereNode`
- Add a `resolve_expression()` to `WhereNode` (unsure what this should
return - requires significant testing with Case/When)
Might be required:
- Add an `output_field=BooleanField()` to `Q`
Tests:
- Adding a subquery containing a Q annotation to a parent query
- Adding a subquery containing a Case/When annotation to a parent query
(if it's not already tested)
- Standard tests that most other annotations already implement
- Nullable fields used in Q annotations
--
Ticket URL: <https://code.djangoproject.com/ticket/27021>
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