title: belongs_to and has_many association problems
body: <p>I have two models <strong>User</strong>  and Hobbie model. Hobbie has already 4 records like music, sports, books and other.</p>
  
  <p>Then I have form where I can create User and there  I can select from checkboxes those Hobbies at least 2.</p>
  
  <p><strong>User.rb</strong></p>
  
  <pre><code>has_many: hobbies
  </code></pre>
  
  <p><strong>Hobbie.rb</strong> </p>
  
  <pre><code>belongs_to :user
  </code></pre>
  
  <p><strong>Form:</strong></p>
  
  <pre><code> &lt;%= form_for(@user,  :remote=&gt;"true",:html =&gt; {:id=&gt;"new_user","data-parsley-validate" =&gt; true,:multipart =&gt; true}, remote: true ,format: :json) do |f| %&gt;
                 ...
  
           &lt;% @hobbies.each do |hobbie| %&gt;
                &lt;li&gt;
  
                  &lt;%= check_box_tag 'hobbie_ids[]',hobbie.id%&gt; &lt;%= h hobbie.name %&gt;                       
                &lt;/li&gt;        
                   &lt;%= f.submit %&gt;
           &lt;% end %&gt;
       &lt;% end %&gt;
  </code></pre>
  
  <p>When I create user with hobbies like music and sports it saves without problems.</p>
  
  <pre><code>t= User.last
  t.hobbies =&gt; "music", "sports"
  </code></pre>
  
  <p><strong>Problem</strong>: When I go for second user and select hobbies like sports and books and save. </p>
  
  <p>Then in console:</p>
  
  <pre><code>t.User.last
  t.hobbies =&gt; "sports" and "books"
  </code></pre>
  
  <p>But for first User there is just "music" left.</p>
  
  <p>I can't figure it out. Do I need to use other assocciation type to get this to work? </p>
  
  <p>Thanks.</p>