post.js

var Post = DS.Model.extend({
  title:      DS.attr('string'),  
  details:    DS.attr('string'),
  user: DS.belongsTo('user')
});

export default Post;

post_serializer.rb

class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :details,  :user_id

end

user.js

var User = DS.Model.extend({
  email: DS.attr('string'),  
  posts: DS.hasMany('post')
});

export default User;

user_serializer.rb

class UserSerializer < ActiveModel::Serializer
  attributes :id, :email
end