ohm-test.rb

require 'ohm'
require 'ohm/json'

module MyModule
    class User < Ohm::Model
      set :posts, :Post
      attribute :name

      def to_hash
        super.merge( :name => name)
      end
    end

    class Post < Ohm::Model
      attribute :value

      def to_hash
        super.merge( :value => value) 
      end
    end
end

Ohm.connect

u= MyModule::User.create(name: 'enrico')
u.posts.add MyModule::Post.create(value: 'something')
u.posts.add MyModule::Post.create(value: 'something else')
p u.posts.to_json