Remove autocast fields in join.

Author: WaVEV

django_mongodb/query.py

@@ -153,7 +153,7 @@ def join(self, compiler, connection):
                 # The pipeline variables to be matched in the pipeline's
                 # expression.
                 "let": {
-                    f"{parent_template}{i}": {"$toString": parent_field}
+                    f"{parent_template}{i}": parent_field
                     for i, parent_field in enumerate(lhs_fields)
                 },
                 "pipeline": [
@@ -167,7 +167,7 @@ def join(self, compiler, connection):
                         "$match": {
                             "$expr": {
                                 "$and": [
-                                    {"$eq": [f"$${parent_template}{i}", {"$toString": field}]}
+                                    {"$eq": [f"$${parent_template}{i}", field]}
                                     for i, field in enumerate(rhs_fields)
                                 ]
                                 + extra_condition