class ActiveModel::Serializer::Associations::HasMany

def serialize(collection, scope, context, options)
  array = collection.map do |item|
    find_serializable(item, scope, context, options).as_json(:root => false)
  end
  { key => array }
end

def serialize_ids(collection, scope)
  # Use pluck or select_columns if available
  # return collection.ids if collection.respond_to?(:ids)
  array = collection.map do |item|
    item.read_attribute_for_serialization(:id)
  end
  { key => array }
end