Ember.js - Cannot read property 'record' of undefined

I am new to ember.js, When I try to edit a post I got this error:-

Uncaught TypeError: Cannot read property 'record' of undefined --- adapter.js:239

Following is @method serialize in adapter.js

  serialize: function(snapshot, options) {
    return get(snapshot.record, 'store').serializerFor(snapshot.modelName).serialize(snapshot, options);
  },

Following is the updateRecord function

App.ApplicationAdapter = DS.RESTAdapter.extend({

  namespace: 'pran/webapp/rest_adapter/api',
    updateRecord: function(store, type, snapshot) {
            var data = this.serialize(snapshot, { includeId: true });
            var id = snapshot.id;
            var url = ['api/update', id].join('/');

            return new Ember.RSVP.Promise(function(resolve, reject) {
              jQuery.ajax({
                type: 'PUT',
                url: url,
                dataType: 'json',
                data: data
              }).then(function(data) {
                Ember.run(null, resolve, data);
              }, function(jqXHR) {
                jqXHR.then = null; // tame jQuery's ill mannered promises
                Ember.run(null, reject, jqXHR);
              });
            });
     }

});

Following is the post controller function

App.PostController = Ember.ObjectController.extend({
  isEditing: false,  
  actions: {

    edit: function() {
      this.set('isEditing', true);
    },
    doneEditing: function() {
      this.set('isEditing', false); 

      var title = $('#title').val();
      var excerpt = $('#excerpt').val();
      var body = $('#body').val();                
      var store = this.get('store');    

      var update_post = store.adapterFor(App.Post).updateRecord('post',{
            title : title,
            excerpt : excerpt,
            body : body
        });
      update_post.save();

    }
  }
});