How do I select which attributes I want for active model serializers relationships

Just make sure to return a hash or array of hashes like so:

def videos    object.listing_videos.collect do |lv|      {        id: lv.video.id,        name: lv.video.name,        wistia_id: lv.video.wistia_id,        duration: lv.video.duration,        wistia_hashed_id: lv.video.wistia_hashed_id,        description: lv.video.description,        thumbnail: lv.video.thumbnail      }    end  end

Instead of defining topics method, it's better to define separate topic serializer with explicitly specifying which attributes do you need to include. This is cleaner, and more maintainable approach then defining topics method.

class PostSerializer < ActiveModel::Serializer  attributes :title  belongs_to :domain  belongs_to :user  # remember to declare TopicSerializer class before you use it  class TopicSerializer < ActiveModel::Serializer    # explicitly tell here which attributes you need from 'topics'    attributes :title  end  has_many :topics, serializer: TopicSerializerend

Again, try to avoid defining methods for relations as much as possible, it's not clean, neither maintainable.