Add docstring

Author: WaVEV

django_mongodb_backend/compiler.py

@@ -553,6 +553,8 @@ def get_lookup_pipeline(self):
         where = self.get_where()
         promote_filters = defaultdict(list)
         for expr in where.children if where and where.connector == AND else ():
+            # Push only basic lookups (no subqueries or complex conditions).
+            # We push by LHS alias to avoid duplication across subqueries.
             if (
                 isinstance(expr, Lookup)
                 and isinstance(expr.lhs, Col)

django_mongodb_backend/query.py

@@ -197,6 +197,7 @@ def _get_reroot_replacements(expression):
     # "pushed down" into the join's $match stage to reduce the volume of
     # joined documents. This only applies to inner joins, as pushing
     # filters into a left join can change the semantics of the result.
+    # LEFT JOINs may rely on null checks to detect missing RHS.
     if pushed_expression and self.join_type == INNER:
         rerooted_replacement = _get_reroot_replacements(pushed_expression)
         extra_conditions.append(