Add AggregateFilter, StringgAgg.as_mql() as per

django/django@4b977a5

Author: timgraham

django_mongodb_backend/aggregates.py

@@ -1,5 +1,13 @@
-from django.db.models.aggregates import Aggregate, Count, StdDev, Variance
-from django.db.models.expressions import Case, Value, When
+from django.db import NotSupportedError
+from django.db.models.aggregates import (
+    Aggregate,
+    AggregateFilter,
+    Count,
+    StdDev,
+    StringAgg,
+    Variance,
+)
+from django.db.models.expressions import Case, Col, Value, When
 from django.db.models.lookups import IsNull
 
 from .query_utils import process_lhs
@@ -16,7 +24,11 @@ def aggregate(
     resolve_inner_expression=False,
     **extra_context,  # noqa: ARG001
 ):
-    if self.filter:
+    # TODO: isinstance(self.filter, Col) works around failure of
+    # aggregation.tests.AggregateTestCase.test_distinct_on_aggregate. Is this
+    # correct?
+    if self.filter is not None and not isinstance(self.filter, Col):
+        # Generate a CASE statement for this aggregate.
         node = self.copy()
         node.filter = None
         source_expressions = node.get_source_expressions()
@@ -31,6 +43,10 @@ def aggregate(
     return {f"${operator}": lhs_mql}
 
 
+def aggregate_filter(self, compiler, connection, **extra_context):
+    return self.condition.as_mql(compiler, connection, **extra_context)
+
+
 def count(self, compiler, connection, resolve_inner_expression=False, **extra_context):  # noqa: ARG001
     """
     When resolve_inner_expression=True, return the MQL that resolves as a
@@ -72,8 +88,14 @@ def stddev_variance(self, compiler, connection, **extra_context):
     return aggregate(self, compiler, connection, operator=operator, **extra_context)
 
 
+def string_agg(self, compiler, connection, **extra_context):  # noqa: ARG001
+    raise NotSupportedError("StringAgg is not supported.")
+
+
 def register_aggregates():
     Aggregate.as_mql = aggregate
+    AggregateFilter.as_mql = aggregate_filter
     Count.as_mql = count
     StdDev.as_mql = stddev_variance
+    StringAgg.as_mql = string_agg
     Variance.as_mql = stddev_variance

django_mongodb_backend/features.py

@@ -99,6 +99,12 @@ class DatabaseFeatures(GISFeatures, BaseDatabaseFeatures):
         # This backend overrides DatabaseCreation.create_test_db() so the
         # deprecation warnings stacklevel points to the wrong file.
         "backends.base.test_creation.TestDbCreationTests.test_serialize_deprecation",
+        # StringAgg is not supported.
+        "aggregation.tests.AggregateTestCase.test_distinct_on_stringagg",
+        "aggregation.tests.AggregateTestCase.test_string_agg_escapes_delimiter",
+        "aggregation.tests.AggregateTestCase.test_string_agg_filter",
+        "aggregation.tests.AggregateTestCase.test_string_agg_filter_in_subquery",
+        "aggregation.tests.AggregateTestCase.test_stringagg_default_value",
     }
     # $bitAnd, #bitOr, and $bitXor are new in MongoDB 6.3.
     _django_test_expected_failures_bitwise = {