Fixed #35275 -- Fixed Meta.constraints validation crash on UniqueConstraint with OpClass().

This also introduces Expression.constraint_validation_compatible that
allows specifying that expression should be ignored during a constraint
validation.

django/contrib/postgres/constraints.py

@@ -1,6 +1,5 @@
 from types import NoneType
 
-from django.contrib.postgres.indexes import OpClass
 from django.core.exceptions import ValidationError
 from django.db import DEFAULT_DB_ALIAS, NotSupportedError
 from django.db.backends.ddl_references import Expressions, Statement, Table
@@ -208,12 +207,10 @@ class ExclusionConstraint(BaseConstraint):
                         if isinstance(expr, F) and expr.name in exclude:
                             return
             rhs_expression = expression.replace_expressions(replacements)
-            # Remove OpClass because it only has sense during the constraint
-            # creation.
-            if isinstance(expression, OpClass):
-                expression = expression.get_source_expressions()[0]
-            if isinstance(rhs_expression, OpClass):
-                rhs_expression = rhs_expression.get_source_expressions()[0]
+            if hasattr(expression, "get_expression_for_validation"):
+                expression = expression.get_expression_for_validation()
+            if hasattr(rhs_expression, "get_expression_for_validation"):
+                rhs_expression = rhs_expression.get_expression_for_validation()
             lookup = PostgresOperatorLookup(lhs=expression, rhs=rhs_expression)
             lookup.postgres_operator = operator
             lookups.append(lookup)

django/contrib/postgres/indexes.py

@@ -244,6 +244,7 @@ class SpGistIndex(PostgresIndex):
 
 class OpClass(Func):
     template = "%(expressions)s %(name)s"
+    constraint_validation_compatible = False
 
     def __init__(self, expression, name):
         super().__init__(expression, name=name)

django/db/models/constraints.py

@@ -6,7 +6,7 @@ from django.core import checks
 from django.core.exceptions import FieldDoesNotExist, FieldError, ValidationError
 from django.db import connections
 from django.db.models.constants import LOOKUP_SEP
-from django.db.models.expressions import Exists, ExpressionList, F, OrderBy, RawSQL
+from django.db.models.expressions import Exists, ExpressionList, F, RawSQL
 from django.db.models.indexes import IndexExpression
 from django.db.models.lookups import Exact
 from django.db.models.query_utils import Q
@@ -644,9 +644,8 @@ class UniqueConstraint(BaseConstraint):
             }
             expressions = []
             for expr in self.expressions:
-                # Ignore ordering.
-                if isinstance(expr, OrderBy):
-                    expr = expr.expression
+                if hasattr(expr, "get_expression_for_validation"):
+                    expr = expr.get_expression_for_validation()
                 expressions.append(Exact(expr, expr.replace_expressions(replacements)))
             queryset = queryset.filter(*expressions)
         model_class_pk = instance._get_pk_val(model._meta)

django/db/models/expressions.py

@@ -180,6 +180,8 @@ class BaseExpression:
     window_compatible = False
     # Can the expression be used as a database default value?
     allowed_default = False
+    # Can the expression be used during a constraint validation?
+    constraint_validation_compatible = True
 
     def __init__(self, output_field=None):
         if output_field is not None:
@@ -484,6 +486,20 @@ class BaseExpression:
             return self.output_field.select_format(compiler, sql, params)
         return sql, params
 
+    def get_expression_for_validation(self):
+        # Ignore expressions that cannot be used during a constraint validation.
+        if not getattr(self, "constraint_validation_compatible", True):
+            try:
+                (expression,) = self.get_source_expressions()
+            except ValueError as e:
+                raise ValueError(
+                    "Expressions with constraint_validation_compatible set to False "
+                    "must have only one source expression."
+                ) from e
+            else:
+                return expression
+        return self
+
 
 @deconstructible
 class Expression(BaseExpression, Combinable):
@@ -1716,6 +1732,7 @@ class Exists(Subquery):
 class OrderBy(Expression):
     template = "%(expression)s %(ordering)s"
     conditional = False
+    constraint_validation_compatible = False
 
     def __init__(self, expression, descending=False, nulls_first=None, nulls_last=None):
         if nulls_first and nulls_last: