Per commit 1426: https://github.com/rails-api/active_model_serializers/pull/1426 – and related discussion, you can see that the default nesting for json and attributes serialization is one level.
If you want deep nesting by default, you can set a configuration property in an active_model_serializer initializer:
ActiveModelSerializers.config.default_includes="**"
For detailed reference from v0.10.6: https://github.com/rails-api/active_model_serializers/blob/v0.10.6/docs/general/adapters.md#include-option