Implement ColPairs.as_mql() to fix regression in some prefetch_related()

Adapt for Django 5.2 which introduced ColPairs.

Author: timgraham

django_mongodb_backend/expressions.py

@@ -8,6 +8,7 @@
 from django.db.models.expressions import (
  Case,
  Col,
+ ColPairs,
  CombinedExpression,
  Exists,
  ExpressionList,
@@ -74,6 +75,13 @@ def col(self, compiler, connection): # noqa: ARG001
  return f"${prefix}{self.target.column}"
 
 
+def col_pairs(self, compiler, connection):
+ cols = self.get_cols()
+ if len(cols) > 1:
+ raise NotSupportedError("ColPairs is not supported.")
+ return cols[0].as_mql(compiler, connection)
+
+
 def combined_expression(self, compiler, connection):
  expressions = [
  self.lhs.as_mql(compiler, connection),
@@ -207,6 +215,7 @@ def value(self, compiler, connection): # noqa: ARG001
 def register_expressions():
  Case.as_mql = case
  Col.as_mql = col
+ ColPairs.as_mql = col_pairs
  CombinedExpression.as_mql = combined_expression
  Exists.as_mql = exists
  ExpressionList.as_mql = process_lhs

django_mongodb_backend/features.py

@@ -90,15 +90,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
  "model_fields_.test_arrayfield.QueryingTests.test_contained_by_subquery",
  # JSONArray not implemented.
  "db_functions.json.test_json_array.JSONArrayTests",
- # Some usage of prefetch_related() raises "ColPairs is not supported."
- "known_related_objects.tests.ExistingRelatedInstancesTests.test_one_to_one_multi_prefetch_related",
- "known_related_objects.tests.ExistingRelatedInstancesTests.test_one_to_one_prefetch_related",
- "prefetch_related.tests.DeprecationTests.test_prefetch_one_level_fallback",
- "prefetch_related.tests.MultiDbTests.test_using_is_honored_fkey",
- "prefetch_related.tests.MultiDbTests.test_using_is_honored_inheritance",
- "prefetch_related.tests.NestedPrefetchTests.test_nested_prefetch_is_not_overwritten_by_related_object",
- "prefetch_related.tests.NullableTest.test_prefetch_nullable",
- "prefetch_related.tests.Ticket19607Tests.test_bug",
  # Value.as_mql() doesn't call output_field.get_db_prep_save():
  # https://github.com/mongodb/django-mongodb-backend/issues/282
  "model_fields.test_jsonfield.TestSaveLoad.test_bulk_update_custom_get_prep_value",

django_mongodb_backend/lookups.py

@@ -1,5 +1,4 @@
 from django.db import NotSupportedError
-from django.db.models.expressions import ColPairs
 from django.db.models.fields.related_lookups import In, RelatedIn
 from django.db.models.lookups import (
  BuiltinLookup,
@@ -35,8 +34,6 @@ def field_resolve_expression_parameter(self, compiler, connection, sql, param):
 
 
 def in_(self, compiler, connection):
- if isinstance(self.lhs, ColPairs):
- raise NotImplementedError("ColPairs is not supported.")
  db_rhs = getattr(self.rhs, "_db", None)
  if db_rhs is not None and db_rhs != connection.alias:
  raise ValueError(