Docstring.

Author: WaVEV

django_mongodb/query.py

@@ -110,27 +110,37 @@ def join(self, compiler, connection):
         # In the lookup stage, the reference to this column doesn't include
         # the collection name.
         rhs_fields.append(rhs.as_mql(compiler, connection))
+    # Check if there are any extra conditions related to the join.
+    # These are specific conditions that must be applied to the join beyond matching
+    # field pairs (e.g., filtering based on additional criteria).
     extra = self.join_field.get_extra_restriction(self.table_alias, self.parent_alias)
     if extra:
         columns = []
         for expr in extra.leaves():
+            # Determine whether the column needs to be transformed or rerouted as part
+            # of the subquery.
             for hand_side in ["lhs", "rhs"]:
                 hand_side_value = getattr(expr, hand_side, None)
                 if isinstance(hand_side_value, Col):
+                    # If the column is not part of the joined table, add it to lhs_fields.
                     if hand_side_value.alias != self.table_name:
                         pos = len(lhs_fields)
                         lhs_fields.append(expr.lhs.as_mql(compiler, connection))
                     else:
                         pos = None
                     columns.append((hand_side_value, pos))
+        # Replace columns in the extra conditions with new column references
+        # based on their rerouted positions in the join pipeline.
         replacements = {}
         for col, parent_pos in columns:
-            # Make all columns in as main collection columns.
             column_target = Col(compiler.collection_name, expr.output_field.__class__())
             if parent_pos is not None:
                 column_target.target.db_column = f"${parent_template}{parent_pos}"
                 column_target.target.set_attributes_from_name(f"${parent_template}{parent_pos}")
+            else:
+                column_target.target = col.target
             replacements[col] = column_target
+        # Apply the transformed expressions in the extra condition.
         extra_condition = [extra.replace_expressions(replacements).as_mql(compiler, connection)]
     else:
         extra_condition = []