[5.0.x] Refs #35638 -- Avoided wrapping expressions with Value in _get_field_value_map() and renamed to _get_field_expression_map().

Backport of 91a038754bb516d29cb79f0fed4025436b5c5346 from main.

django/contrib/postgres/constraints.py

@@ -183,7 +183,7 @@ class ExclusionConstraint(BaseConstraint):
 
     def validate(self, model, instance, exclude=None, using=DEFAULT_DB_ALIAS):
         queryset = model._default_manager.using(using)
-        replacement_map = instance._get_field_value_map(
+        replacement_map = instance._get_field_expression_map(
             meta=model._meta, exclude=exclude
         )
         replacements = {F(field): value for field, value in replacement_map.items()}

django/db/models/base.py

@@ -1244,12 +1244,17 @@ class Model(AltersData, metaclass=ModelBase):
             setattr(self, cachename, obj)
         return getattr(self, cachename)
 
-    def _get_field_value_map(self, meta, exclude=None):
+    def _get_field_expression_map(self, meta, exclude=None):
         if exclude is None:
             exclude = set()
         meta = meta or self._meta
         field_map = {
-            field.name: Value(getattr(self, field.attname), field)
+            field.name: (
+                value
+                if (value := getattr(self, field.attname))
+                and hasattr(value, "resolve_expression")
+                else Value(value, field)
+            )
             for field in meta.local_concrete_fields
             if field.name not in exclude and not field.generated
         }

django/db/models/constraints.py

@@ -124,7 +124,7 @@ class CheckConstraint(BaseConstraint):
         return schema_editor._delete_check_sql(model, self.name)
 
     def validate(self, model, instance, exclude=None, using=DEFAULT_DB_ALIAS):
-        against = instance._get_field_value_map(meta=model._meta, exclude=exclude)
+        against = instance._get_field_expression_map(meta=model._meta, exclude=exclude)
         try:
             if not Q(self.check).check(against, using=using):
                 raise ValidationError(
@@ -423,7 +423,7 @@ class UniqueConstraint(BaseConstraint):
                         return
             replacements = {
                 F(field): value
-                for field, value in instance._get_field_value_map(
+                for field, value in instance._get_field_expression_map(
                     meta=model._meta, exclude=exclude
                 ).items()
             }
@@ -454,7 +454,9 @@ class UniqueConstraint(BaseConstraint):
                     code=self.violation_error_code,
                 )
         else:
-            against = instance._get_field_value_map(meta=model._meta, exclude=exclude)
+            against = instance._get_field_expression_map(
+                meta=model._meta, exclude=exclude
+            )
             try:
                 if (self.condition & Exists(queryset.filter(self.condition))).check(
                     against, using=using