How to serialize hash values with active-model-serializers

Question

I have a serializer FooSerializer serializing my model Foo. The serializer works. But in one of my controller I do a render json: { key: [foo_1, foo_2] } and the instances foo_1 and foo_2 are not serialized with FooSerializer (only a to_json is called I guess).

How to do it with a cleaner way than calling FooSerializer specifically on each item of the array ?

Thanks !