Docstring and refactor.

Author: WaVEV

django_mongodb/compiler.py

@@ -31,10 +31,10 @@ class SQLCompiler(compiler.SQLCompiler):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.aggregation_pipeline = None
+        # Dictionary mapping parent variable names to their corresponding indices.
+        self.column_mapping = {}
         # A list of OrderBy objects for this query.
         self.order_by_objs = None
-        # Subquery parent compiler.
-        self.column_mapping = {}
         self.subqueries = []
 
     def _unfold_column(self, col):

django_mongodb/expressions.py

@@ -106,7 +106,10 @@ def query(self, compiler, connection, lookup_name=None):
     from_table = next(
         e.table_name for alias, e in self.alias_map.items() if self.alias_refcount[alias]
     )
-    subquery.lookup_data = {
+    # To perform a subquery, a $lookup stage is added.
+    # This stage encapsulates the entire subquery pipeline within a $lookup operation.
+    # The "let" clause defines the variables needed to bridge the main collection with the subquery.
+    subquery.subquery_lookup = {
         "as": table_output,
         "from": from_table,
         "let": {

django_mongodb/query.py

@@ -56,7 +56,8 @@ def __init__(self, compiler):
         self.aggregation_pipeline = compiler.aggregation_pipeline
         self.extra_fields = None
         self.combinator_pipeline = None
-        self.lookup_data = None
+        # Stage that encapsulates the pipeline for performing a nested subquery within a $lookup.
+        self.subquery_lookup = None
 
     def __repr__(self):
         return f"<MongoQuery: {self.mongo_query!r} ORDER {self.ordering!r}>"
@@ -96,10 +97,10 @@ def get_pipeline(self):
             pipeline.append({"$skip": self.query.low_mark})
         if self.query.high_mark is not None:
             pipeline.append({"$limit": self.query.high_mark - self.query.low_mark})
-        if self.lookup_data:
-            table_output = self.lookup_data["as"]
+        if self.subquery_lookup:
+            table_output = self.subquery_lookup["as"]
             pipeline = [
-                {"$lookup": {**self.lookup_data, "pipeline": pipeline}},
+                {"$lookup": {**self.subquery_lookup, "pipeline": pipeline}},
                 {
                     "$set": {
                         table_output: {