fix QuerySet.annotate() with F() expression

Author: timgraham

.github/workflows/test-python.yml

@@ -68,6 +68,7 @@ jobs:
         run: >
           python3 django_repo/tests/runtests.py --settings mongodb_settings -v 2
           annotations.tests.NonAggregateAnnotationTestCase.test_basic_annotation
+          annotations.tests.NonAggregateAnnotationTestCase.test_basic_f_annotation
           auth_tests.test_models.UserManagerTestCase
           backends.base.test_base.DatabaseWrapperTests
           basic

django_mongodb/query.py

@@ -88,10 +88,14 @@ def get_cursor(self):
         fields = {}
         for name, expr in self.columns or []:
             try:
-                fields[expr.target.column] = 1
+                column = expr.target.column
             except AttributeError:
                 # Generate the MQL for an annotation.
                 fields[name] = expr.as_mql_agg(self.compiler, self.connection)
+            else:
+                # If name != column, then this is an annotatation referencing
+                # another column.
+                fields[name] = 1 if name == column else f"${column}"
         pipeline = []
         if self.mongo_query:
             pipeline.append({"$match": self.mongo_query})