Ember-data and MongoDB, how to handle _id

问题:

I'm using ember-data with rails and MongoDB and am having problem with the way IDs are stored in MongoDB - in a _id field.

Ember-data will use id as the default field for ID so I tried to override it like this:

App.User = DS.Model.extend
    primaryKey: "_id"
    name: DS.attr "string"
    image: DS.attr "string"

This seems to work most of the time but in some instances I get exceptions from ember saying:

Uncaught Error: assertion failed: Your server returned a hash with the key _id but you have no mappings

I suspect this might be a bug in ember-data because it's still heavily under development, but I was trying to find a way to get to map _id to id on the server side in rails? I'm using mongoid to do the mongo mapping.

回答1:

If you are using Mongoid here is a solution that makes it so you don't have to add a method def id; object._id.to_s; end to every serializer

Add the following Rails initializer

Mongoid 3.x

module Moped
  module BSON
    class ObjectId
      alias :to_json :to_s
      alias :as_json :to_s
    end
  end
end

Mongoid 4

module BSON
  class ObjectId
    alias :to_json :to_s
    alias :as_json :to_s
  end
end

Active Model Serializer for Building

class BuildingSerializer < ActiveModel::Serializer
  attributes :id, :name
end

Resulting JSON

{
  "buildings": [
    {"id":"5338f70741727450f8000000","name":"City Hall"},    
    {"id":"5338f70741727450f8010000","name":"Firestation"}
  ]
}

This is a monkey patch suggested by brentkirby and updated for Mongoid 4 by arthurnn

回答2:

An other way could be to use (if possible for you) the ActiveModel::Serializer. (I think it should be close to rabl (?))

From the ember-data gihtub: https://github.com/emberjs/data:
Out of the box support for Rails apps that follow the active_model_serializers gem's conventions

When we began with ember-data we were crafting as_json(), but using the gem is definitely better :)

回答3:

Ahh, instead of including _id in your JSON, you could craft the JSON to instead use the id method rather than the _id attribute. Ways:

You could use rabl, and the JSON could be like:

object @user 
attributes :id, :email
node(:full_name) {|user| "#{user.first_name} #{user.last_name}"}

You could also craft the as_json method

class User
  def as_json(args={})
    super args.merge(:only => [:email], :methods => [:id, :full_name])
  end
end

回答4:

I had a similar problem using ember.js with ember-resource and couchdb, which also stores it's IDs as _id.

As solution to this problem I defined a superclass for all my model classes containing a computed property to duplicate _id into id like this:

// get over the fact that couchdb uses _id, ember-resource uses id
id: function(key, value) {
    // map _id (couchdb) to id (ember)
    if (arguments.length === 1) {
        return this.get('_id');
    }
    else {
        this.set('_id', value);
        return value;
    }
}.property('_id').cacheable()

Maybe this could solve your problem too?

回答5:

The best way is to use ActiveModel::Serializers. Since we are using Mongoid, you will need to add an include statement like that (see this gist from benedikt):

# config/initializers/active_model_serializers.rb
Mongoid::Document.send(:include, ActiveModel::SerializerSupport)
Mongoid::Criteria.delegate(:active_model_serializer, :to => :to_a)

And then include your serializer. Something like that:

# app/serializers/user_serializer.rb
class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :email
  def id
    object._id
  end 
end

This fixes the _id problem

回答6:

The second part of joscas's answer fixed the id issue for me with Rails4/Ruby2 except I had to .to_s the _id.

class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :email
  def id
    object._id.to_s
  end
end

回答7:

If you use Mongoid3, here is the monkey patch may work for you.

https://gist.github.com/4700909

回答8:

I don't know exactly when this was added, but you can just tell Ember-Data that the primaryKey is _id:

DS.RESTAdapter.extend({
  serializer: DS.RESTSerializer.extend({
    primaryKey: '_id'
  })
});

回答9:

Although the question is quite old but i still think my answer could help others:

If you are using ActiveModelSerializer then you just need to do this :

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

It works all fine. I am using emberjs on the front end btw.