Add docstring.

Author: WaVEV

django_mongodb/compiler.py

@@ -26,26 +26,39 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self._group_pipeline = None
 
-    def _get_group_alias_column(self, col, annotation_group_idx):
-        """Generate alias and replacement for group columns."""
+    def _get_group_alias_column(self, expr, annotation_group_idx):
+        """
+        Generate a dummy field for use in ids fields in $group.
+
+        If the column is a composite expression (not a single column),
+        it creates an auxiliary field and pass it through the flow.
+        When a foreign field is needed in the ids, escape the dot (.) using GROUP_SEPARATOR.
+        """
         replacement = None
-        if not isinstance(col, Col):
+        if isinstance(expr, Col):
+            col = expr
+        else:
             alias = f"__annotation_group{next(annotation_group_idx)}"
-            col_expr = self._get_column_from_expression(col, alias)
+            col_expr = self._get_column_from_expression(expr, alias)
             replacement = col_expr
             col = col_expr
         if self.collection_name == col.alias:
             return col.target.column, replacement
         return f"{col.alias}{self.GROUP_SEPARATOR}{col.target.column}", replacement
 
     def _get_column_from_expression(self, expr, alias):
-        """Get column target from expression."""
+        """
+        Create a new column with the specified output type and alias to hold the aggregate value.
+
+        This function generates a column target from the given expression, setting the column's
+        output type and assigning the provided alias to the column.
+        """
         column_target = expr.output_field.__class__()
         column_target.db_column = alias
         column_target.set_attributes_from_name(alias)
         return Col(self.collection_name, column_target)
 
-    def _prepare_expressions_for_pipeline(self, expression, target, count):
+    def _prepare_expressions_for_pipeline(self, expression, target, annotation_group_idx):
         """
         Prepare expressions for the aggregation pipeline.
 
@@ -64,7 +77,9 @@ def _prepare_expressions_for_pipeline(self, expression, target, count):
         replacements = {}
         group = {}
         for sub_expr in self._get_aggregate_expressions(expression):
-            alias = f"__aggregation{next(count)}" if sub_expr != expression else target
+            alias = (
+                f"__aggregation{next(annotation_group_idx)}" if sub_expr != expression else target
+            )
             column_target = sub_expr.output_field.__class__()
             column_target.db_column = alias
             column_target.set_attributes_from_name(alias)