[Django] #34871: Validation fails for models with UniqueConstraints that include Case statements

[Django]

#34871: Validation fails for models with UniqueConstraints that include Case
statements
-------------------------------------+-------------------------------------
Reporter: Andrew | Owner: nobody
Roberts |
Type: Bug | Status: new
Component: Database | Version: 4.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Consider the following model where we want to guarantee that there is only
one active profile per email address:

{{{
class Profile(models.Model):
active = models.BooleanField()
email = models.CharField(max_length=255)

class Meta:
constraints = [
models.UniqueConstraint(
Case(When(active=True, then=F("email"))),
name="unique_active_email",
)
]
app_label = "test"
}}}

Model validation indirectly calls constraint.validate(), but to simulate
this we can execute:

{{{
constraint = Profile._meta.constraints[0]
constraint.validate(Profile, Profile(active=True,
email="te...@example.com"))
}}}

This fails with:
{{{
---------------------------------------------------------------------------
AttributeError Traceback (most recent call
last)
Cell In[21], line 1
----> 1 constraint.validate(Profile, Profile(active=True,
email="te...@example.com"))

File /usr/local/lib/python3.11/site-
packages/django/db/models/constraints.py:346, in
UniqueConstraint.validate(self, model, instance, exclude, using)
344 if isinstance(expr, OrderBy):
345 expr = expr.expression
--> 346 expressions.append(Exact(expr,
expr.replace_expressions(replacements)))
347 queryset = queryset.filter(*expressions)
348 model_class_pk = instance._get_pk_val(model._meta)

File /usr/local/lib/python3.11/site-
packages/django/db/models/expressions.py:401, in
BaseExpression.replace_expressions(self, replacements)
398 clone = self.copy()
399 source_expressions = clone.get_source_expressions()
400 clone.set_source_expressions(
--> 401 [
402 expr.replace_expressions(replacements) if expr else None
403 for expr in source_expressions
404 ]
405 )
406 return clone

File /usr/local/lib/python3.11/site-
packages/django/db/models/expressions.py:402, in <listcomp>(.0)
398 clone = self.copy()
399 source_expressions = clone.get_source_expressions()
400 clone.set_source_expressions(
401 [
--> 402 expr.replace_expressions(replacements) if expr else None
403 for expr in source_expressions
404 ]
405 )
406 return clone

File /usr/local/lib/python3.11/site-
packages/django/db/models/expressions.py:401, in
BaseExpression.replace_expressions(self, replacements)
398 clone = self.copy()
399 source_expressions = clone.get_source_expressions()
400 clone.set_source_expressions(
--> 401 [
402 expr.replace_expressions(replacements) if expr else None
403 for expr in source_expressions
404 ]
405 )
406 return clone

File /usr/local/lib/python3.11/site-
packages/django/db/models/expressions.py:402, in <listcomp>(.0)
398 clone = self.copy()
399 source_expressions = clone.get_source_expressions()
400 clone.set_source_expressions(
401 [
--> 402 expr.replace_expressions(replacements) if expr else None
403 for expr in source_expressions
404 ]
405 )
406 return clone

AttributeError: 'Q' object has no attribute 'replace_expressions'
}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/34871>
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