How to create record with belongsTo association with ember.js front end and rails back end

I'm trying to create a record that has a belongs_to association. I have a rails back end with an ember front end.

I'm able to create users, but can't figure out how to create posts for users, i.e.,

# rails/db/seeds.rb

user.posts.create(title: "bonjour")

Rails

Here are the rails models

class Post < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :posts, :dependent => :destroy
end

Here is the posts controller:

class Api::PostsController < ApplicationController

...

  def update
    post = Post.find(params[:id])
    post.title = params[:post]['title']
    post.save!
    render json: post
  end

  def create
    user = User.find(params[:id])
    post = user.posts.create(title: params[:title])
    render json: post
  end

end

Here are the serializers' files

# app/serializers/posts_serializer.rb
class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :user_id
end

# app/serializers/user_serializer.rb
class UserSerializer < ActiveModel::Serializer
  embed :ids, include: true

  attributes :id, :name
  has_many :posts
end

Ember

Here are the Ember models

// app/models/post.js

export default DS.Model.extend({
  title: DS.attr('string'),
  user: DS.belongsTo('user', { async: true})
});

// app/models/user.js

export default DS.Model.extend({
  name: DS.attr('string'),
  posts: DS.hasMany('post')
});

Here is the create.js file for posts

// app/routes/posts/create.js

export default Ember.Route.extend({

model: function () {
    return this.store.createRecord('post', {
        title: ''
    });
},

actions: {
    create: function () {
        var my_model = this.controller.get('model');
        my_model.save();
        this.transitionTo('users.show', my_model);
    }
  }
});

Here is the create.hbs template

app/templates/posts/create.hbs

<h4 class="create-new-post">Create New Post</h4>

{{input value=title class='post-title'}}

<p><button type='submit' {{action 'create'}} class='commit-post-creation'>Submit</button></p>

And lastly, here is the app/router.js file

...    

Router.map(function() {
  this.resource('users', function () {
    this.route('show', {path: ':user_id'});
    this.route('edit', {path: ':user_id/edit'});
    this.route('create', {path: 'create'});
  });

  this.resource('posts', function () {
    this.route('show', {path: ':post_id'});
    this.route('edit', {path: ':post_id/edit'});
    this.route('create', {path: 'create'});
  });

});

... 

When I try to create a post, I get the error:

POST http://localhost:4200/api/posts 404 (Not Found)

The post shows up at first, but there is no user associated with the post, and then it disappears when the page refreshes.

Thanks. Any help would be really appreciated. I'm just learning Ember, so sorry about the massive post.