Return default collection when no table is given.

Author: WaVEV

django_mongodb/compiler.py

@@ -435,11 +435,17 @@ def project_field(column):
 
     @cached_property
     def collection_name(self):
-        base_table = next(
-            v
-            for k, v in self.query.alias_map.items()
-            if isinstance(v, BaseTable) and self.query.alias_refcount[k]
-        )
+        try:
+            base_table = next(
+                v
+                for k, v in self.query.alias_map.items()
+                if isinstance(v, BaseTable) and self.query.alias_refcount[k]
+            )
+        except StopIteration:
+            query = self.query_class(self)
+            query.aggregation_pipeline = [{"$facet": {"__null": []}}]
+            self.subqueries.insert(0, query)
+            return "__null"
         return base_table.table_alias or base_table.table_name
 
     @cached_property