Association error within Rails 4 app with mongoid + simple_form + devise

Question

As mentioned on title, I made a Rails 4 app that contains

• mongoid (5.1.3)
• devise (4.2.0)
• simple_form (3.2.1)

I set up my app with commands below:

rails g mongoid:config
rails g simple_form:install --bootstrap
rails g devise:install
rails g devise User
rails g scaffold post title content:text user:references

I add the code below in user.rb model file to make association between two models:

  embeds_many :posts

When I run app and create a user on it, then I visit http://localhost:3000/posts/new to create a new post, I got the error below:

Started GET "/posts/new" for 127.0.0.1 at 2016-07-29 19:54:21 +0300
Processing by PostsController#new as HTML
MONGODB | Adding localhost:27017 to the cluster.
MONGODB | localhost:27017 | mongoblog_development.find | STARTED | {"find"=>"users", "filter"=>{}}
MONGODB | localhost:27017 | mongoblog_development.find | SUCCEEDED | 0.0009257450000000001s
  Rendered posts/_form.html.erb (134.4ms)
  Rendered posts/new.html.erb within layouts/application (140.6ms)
Completed 500 Internal Server Error in 144ms

ActionView::Template::Error (undefined method `user_ids' for #<Post _id: 579b8a3d419a8f180afbb798, title: nil, content: nil>
Did you mean?  user?):
    4:   <div class="form-inputs">
    5:     <%= f.input :title %>
    6:     <%= f.input :content %>
    7:     <%= f.association :user %>
    8:   </div>
    9: 
   10:   <div class="form-actions">
  app/views/posts/_form.html.erb:7:in `block in _app_views_posts__form_html_erb__2731111319355307354_47423040086160'
  app/views/posts/_form.html.erb:1:in `_app_views_posts__form_html_erb__2731111319355307354_47423040086160'
  app/views/posts/new.html.erb:3:in `_app_views_posts_new_html_erb__3061427264044886497_70253725122920'

I searched it on search engine and here but there was nothing related with this issue. How can I fix it?

TEMPORARY SOLUTION:

Replacing embeds_many and embedded_in associations in model files with has_many and belongs_to makes the app work. By this replacement, it is possible to access Posts belongs to User or Users Post. I wonder whether it's possible to make association in first situation, using embeds_many and embedded_in.