Raise exception when subqueries are across different databases.

Author: WaVEV

django_mongodb/query_utils.py

@@ -27,6 +27,12 @@ def process_lhs(node, compiler, connection):
 
 def process_rhs(node, compiler, connection):
     rhs = node.rhs
+    db_rhs = getattr(rhs, "_db", None)
+    if db_rhs is not None and db_rhs != connection.alias:
+        raise ValueError(
+            "Subqueries aren't allowed across different databases. Force "
+            "the inner query to be evaluated using `list(inner_query)`."
+        )
     if hasattr(rhs, "as_mql"):
         if getattr(rhs, "subquery", False):
             value = rhs.as_mql(compiler, connection, lookup_name=node.lookup_name)