diff --git a/django_mongodb/compiler.py b/django_mongodb/compiler.py
index 0b724114..73d01beb 100644
--- a/django_mongodb/compiler.py
+++ b/django_mongodb/compiler.py
@@ -434,13 +434,16 @@ def project_field(column):
         )
 
     @cached_property
-    def collection_name(self):
-        base_table = next(
+    def base_table(self):
+        return next(
             v
             for k, v in self.query.alias_map.items()
             if isinstance(v, BaseTable) and self.query.alias_refcount[k]
         )
-        return base_table.table_alias or base_table.table_name
+
+    @cached_property
+    def collection_name(self):
+        return self.base_table.table_alias or self.base_table.table_name
 
     @cached_property
     def collection(self):
@@ -469,6 +472,7 @@ def get_combinator_queries(self):
                         )
                     )
                 compiler_.pre_sql_setup()
+                compiler_.column_indices = self.column_indices
                 columns = compiler_.get_columns()
                 parts.append((compiler_.build_query(columns), compiler_, columns))
             except EmptyResultSet:
@@ -496,7 +500,12 @@ def get_combinator_queries(self):
             # Combine query with the current combinator pipeline.
             if combinator_pipeline:
                 combinator_pipeline.append(
-                    {"$unionWith": {"coll": compiler_.collection_name, "pipeline": inner_pipeline}}
+                    {
+                        "$unionWith": {
+                            "coll": compiler_.base_table.table_name,
+                            "pipeline": inner_pipeline,
+                        }
+                    }
                 )
             else:
                 combinator_pipeline = inner_pipeline
diff --git a/django_mongodb/expressions.py b/django_mongodb/expressions.py
index a95e8b1c..626f7711 100644
--- a/django_mongodb/expressions.py
+++ b/django_mongodb/expressions.py
@@ -122,39 +122,44 @@ def query(self, compiler, connection, lookup_name=None):
     if lookup_name in ("in", "range"):
         if subquery.aggregation_pipeline is None:
             subquery.aggregation_pipeline = []
-        subquery.aggregation_pipeline.extend(
-            [
-                {
-                    "$facet": {
-                        "group": [
+        wrapping_result_pipeline = [
+            {
+                "$facet": {
+                    "group": [
+                        {
+                            "$group": {
+                                "_id": None,
+                                "tmp_name": {
+                                    "$addToSet": expr.as_mql(subquery_compiler, connection)
+                                },
+                            }
+                        }
+                    ]
+                }
+            },
+            {
+                "$project": {
+                    field_name: {
+                        "$ifNull": [
                             {
-                                "$group": {
-                                    "_id": None,
-                                    "tmp_name": {
-                                        "$addToSet": expr.as_mql(subquery_compiler, connection)
-                                    },
+                                "$getField": {
+                                    "input": {"$arrayElemAt": ["$group", 0]},
+                                    "field": "tmp_name",
                                 }
-                            }
+                            },
+                            [],
                         ]
                     }
-                },
-                {
-                    "$project": {
-                        field_name: {
-                            "$ifNull": [
-                                {
-                                    "$getField": {
-                                        "input": {"$arrayElemAt": ["$group", 0]},
-                                        "field": "tmp_name",
-                                    }
-                                },
-                                [],
-                            ]
-                        }
-                    }
-                },
-            ]
-        )
+                }
+            },
+        ]
+        # If the subquery is a combinator, wrap the result at the end of the
+        # combinator pipeline...
+        if subquery.query.combinator:
+            subquery.combinator_pipeline.extend(wrapping_result_pipeline)
+        # ... otherwise put at the end of subquery's pipeline.
+        else:
+            subquery.aggregation_pipeline.extend(wrapping_result_pipeline)
         # Erase project_fields since the required value is projected above.
         subquery.project_fields = None
     compiler.subqueries.append(subquery)
diff --git a/django_mongodb/features.py b/django_mongodb/features.py
index 3be98779..d9cb8b89 100644
--- a/django_mongodb/features.py
+++ b/django_mongodb/features.py
@@ -73,11 +73,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # Connection creation doesn't follow the usual Django API.
         "backends.tests.ThreadTests.test_pass_connection_between_threads",
         "backends.tests.ThreadTests.test_default_connection_thread_local",
-        # Union as subquery is not mapping the parent parameter and collections:
-        # https://github.com/mongodb-labs/django-mongodb/issues/156
-        "queries.test_qs_combinators.QuerySetSetOperationTests.test_union_in_subquery_related_outerref",
-        "queries.test_qs_combinators.QuerySetSetOperationTests.test_union_in_subquery",
-        "queries.test_qs_combinators.QuerySetSetOperationTests.test_union_in_with_ordering",
         # ObjectId type mismatch in a subquery:
         # https://github.com/mongodb-labs/django-mongodb/issues/161
         "queries.tests.RelatedLookupTypeTests.test_values_queryset_lookup",