ActiveRecordで取得したデータをカスタマイズしてJSONで返却

model.rb

class Model < ActiveRecord::Base
  has_many :users
  def as_json(options={})
    super.as_json(options).merge({users: self.users})
   end
end

model_controller.rb

class ModelController < ApplicationController
   # usersを含めたjsonの返却
   def index
        render json: { data: Model.all }
   end
end

users.rb

class User < ActiveRecord::Base
  belongs_to :model
end