Move field check to validate_constraints()

django/db/models/base.py

@@ -1441,7 +1441,15 @@ class Model(metaclass=ModelBase):
                 try:
                     constraint.validate(model_class, self, exclude=exclude, using=using)
                 except ValidationError as e:
-                    if e.code == "unique" and len(constraint.fields) == 1:
+                    if (
+                        e.code == "unique"
+                        and len(constraint.fields) == 1
+                        and (
+                            constraint.condition is None
+                            or set(constraint.fields)
+                            == constraint.condition.referenced_base_fields
+                        )
+                    ):
                         errors.setdefault(constraint.fields[0], []).append(e)
                     else:
                         errors = e.update_error_dict(errors)

django/db/models/constraints.py

@@ -364,13 +364,8 @@ class UniqueConstraint(BaseConstraint):
                 if (self.condition & Exists(queryset.filter(self.condition))).check(
                     against, using=using
                 ):
-                    code = (
-                        "unique"
-                        if self.condition.referenced_base_fields == set(self.fields)
-                        else None
-                    )
                     raise ValidationError(
-                        message=self.get_violation_error_message(), code=code
+                        message=self.get_violation_error_message(), code="unique"
                     )
             except FieldError:
                 pass