Fixed #33902 -- Fixed Meta.constraints validation crash with F() expressions.

Thanks Adam Zahradník for the report.

Bug in 667105877e.

django/db/models/constraints.py

@@ -326,9 +326,12 @@ class UniqueConstraint(BaseConstraint):
             # Ignore constraints with excluded fields.
             if exclude:
                 for expression in self.expressions:
-                    for expr in expression.flatten():
-                        if isinstance(expr, F) and expr.name in exclude:
-                            return
+                    if hasattr(expression, "flatten"):
+                        for expr in expression.flatten():
+                            if isinstance(expr, F) and expr.name in exclude:
+                                return
+                    elif isinstance(expression, F) and expression.name in exclude:
+                        return
             replacement_map = instance._get_field_value_map(
                 meta=model._meta, exclude=exclude
             )

django/db/models/expressions.py

@@ -393,9 +393,7 @@ class BaseExpression:
         clone = self.copy()
         clone.set_source_expressions(
             [
-                references_map.get(expr.name, expr)
-                if isinstance(expr, F)
-                else expr.replace_references(references_map)
+                expr.replace_references(references_map)
                 for expr in self.get_source_expressions()
             ]
         )
@@ -810,6 +808,9 @@ class F(Combinable):
     ):
         return query.resolve_ref(self.name, allow_joins, reuse, summarize)
 
+    def replace_references(self, references_map):
+        return references_map.get(self.name, self)
+
     def asc(self, **kwargs):
         return OrderBy(self, **kwargs)
 

docs/releases/4.1.1.txt

@@ -29,3 +29,6 @@ Bugfixes
 
 * Fixed a regression in Django 4.1 that caused a migration crash on SQLite
   3.35.5+ when removing an indexed field (:ticket:`33899`).
+
+* Fixed a bug in Django 4.1 that caused a crash of model validation on
+  ``UniqueConstraint()`` with field names in ``expressions`` (:ticket:`33902`).

tests/constraints/tests.py

@@ -685,6 +685,20 @@ class UniqueConstraintTests(TestCase):
             exclude={"color"},
         )
 
+    def test_validate_expression_str(self):
+        constraint = models.UniqueConstraint("name", name="name_uniq")
+        msg = "Constraint “name_uniq” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            constraint.validate(
+                UniqueConstraintProduct,
+                UniqueConstraintProduct(name=self.p1.name),
+            )
+        constraint.validate(
+            UniqueConstraintProduct,
+            UniqueConstraintProduct(name=self.p1.name),
+            exclude={"name"},
+        )
+
     def test_name(self):
         constraints = get_constraints(UniqueConstraintProduct._meta.db_table)
         expected_name = "name_color_uniq"