id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
33902	UniqueConstraint's validation crashes with string expressions.	Adam Zahradník	Mariusz Felisiak	"When using UniqueConstraint with field names in `expressions` in Django 4.1, an error is raised when the model is saved:

{{{
Traceback (most recent call last):
  File ""/usr/local/lib/python3.10/site-packages/django/core/handlers/exception.py"", line 55, in inner
    response = get_response(request)
  File ""/usr/local/lib/python3.10/site-packages/django/core/handlers/base.py"", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/contrib/admin/options.py"", line 686, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/utils/decorators.py"", line 133, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/views/decorators/cache.py"", line 62, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/contrib/admin/sites.py"", line 242, in inner
    return view(request, *args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/contrib/admin/options.py"", line 1890, in add_view
    return self.changeform_view(request, None, form_url, extra_context)
  File ""/usr/local/lib/python3.10/site-packages/django/utils/decorators.py"", line 46, in _wrapper
    return bound_method(*args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/utils/decorators.py"", line 133, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File ""/usr/local/lib/python3.10/site-packages/django/contrib/admin/options.py"", line 1750, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
  File ""/usr/local/lib/python3.10/site-packages/django/contrib/admin/options.py"", line 1796, in _changeform_view
    form_validated = form.is_valid()
  File ""/usr/local/lib/python3.10/site-packages/django/forms/forms.py"", line 205, in is_valid
    return self.is_bound and not self.errors
  File ""/usr/local/lib/python3.10/site-packages/django/forms/forms.py"", line 200, in errors
    self.full_clean()
  File ""/usr/local/lib/python3.10/site-packages/django/forms/forms.py"", line 439, in full_clean
    self._post_clean()
  File ""/usr/local/lib/python3.10/site-packages/django/forms/models.py"", line 492, in _post_clean
    self.instance.full_clean(exclude=exclude, validate_unique=False)
  File ""/usr/local/lib/python3.10/site-packages/django/db/models/base.py"", line 1491, in full_clean
    self.validate_constraints(exclude=exclude)
  File ""/usr/local/lib/python3.10/site-packages/django/db/models/base.py"", line 1442, in validate_constraints
    constraint.validate(model_class, self, exclude=exclude, using=using)
  File ""/usr/local/lib/python3.10/site-packages/django/db/models/constraints.py"", line 329, in validate
    for expr in expression.flatten():
AttributeError: 'F' object has no attribute 'flatten'
}}}

The culprit is most probably the new constraint validation (call to self.validate_constraints in the traceback).

This is excerpt from the model in question:

{{{
class Model:
    class Meta:
        constraints = (
            UniqueConstraint(
                ""field1"",
                ""field2"",
                name=""some_constraint_name"",
            ),
        )
}}}

Please note, that I am not using the `fields` keyword argument, but the positional `expressions`.

I can confirm that the same model works in Django 4.0.7."	Bug	closed	Database layer (models, ORM)	4.1	Release blocker	fixed		Gagaro	Accepted	1	0	0	0	0	0