docstring.

Author: WaVEV

django_mongodb/query.py

@@ -157,13 +157,18 @@ def join(self, compiler, connection):
     lookup_pipeline = [
         {
             "$lookup": {
+                # The right-hand table to join.
                 "from": self.table_name,
+                # Define the current pipeline variables to be used in the match
+                # expression of the pipeline.
                 "let": {
                     f"{parent_template}{i}": parent_field
                     for i, parent_field in enumerate(lhs_fields)
                 },
                 "pipeline": [
                     {
+                        # Match all the conditions: self.table_name.field1 = parent_table.field1
+                        # and self.table_name.field2 = parent_table.field2 and ...
                         "$match": {
                             "$expr": {
                                 "$and": [
@@ -174,6 +179,7 @@ def join(self, compiler, connection):
                         }
                     }
                 ],
+                # Rename the output as table_alias.
                 "as": self.table_alias,
             }
         },