title: can't write unknown attribute `scrapbook_entry_id'
body: <p>Attempting to add data to a join table of scrapbook_entries which has_one :scrapbook and has_one :recipe. </p>
  
  <p>:recipe and :scrapbook already exist. I am trying to add them to link them with the scrapbook_entries table.</p>
  
  <p>form_for adding to scrapbook_entries table:</p>
  
  <pre><code>&lt;%= form_for(@scrapbook_entry, :url =&gt; scrapbook_entries_path(params[:id])) do |f| %&gt;
    &lt;%= render 'shared/error_messages', object: f.object %&gt;
    &lt;div class="field"&gt;
      &lt;%=f.select(:scrapbook_id, current_user.scrapbooks.collect {|p| [ p.name, p.id ] },  {prompt: 'Select Scrapbook...'})%&gt;
      &lt;%= f.hidden_field :recipe_id, :value =&gt; @recipe.id %&gt;
    &lt;/div&gt;
    &lt;%= f.submit "Save", class: "btn btn-large btn-primary" %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>scrapbook_entries_controller:</p>
  
  <pre><code>def create
      @recipe = Recipe.find(params[:scrapbook_entry][:recipe_id])
      @scrapbook = current_user.scrapbooks.find(params[:scrapbook_entry][:scrapbook_id])
  
      @entry = @scrapbook.scrapbook_entries.build(scrapbook: @scrapbook)
      if @entry.save
          flash[:success] = "Added '#{@recipe.name}' to scrapbook '#{@scrapbook.name}'"
      else
          flash[:error] = "Could not add to scrapbook"
      end
      redirect_to @recipe
  end
  </code></pre>
  
  <p>scrapbook.rb</p>
  
  <pre><code>has_many :recipes, through: :scrapbook_entries
  has_many :scrapbook_entries
  </code></pre>
  
  <p>recipe.rb</p>
  
  <pre><code>has_many :scrapbooks, through: :scrapbook_entries
  </code></pre>
  
  <p>scrapbook_entry.rb</p>
  
  <pre><code>has_one :recipe
  has_one :scrapbook
  </code></pre>
  
  <p>On submitting the form to the controller I am getting a error:</p>
  
  <pre><code>can't write unknown attribute `scrapbook_entry_id'
  </code></pre>
  
  <p>Can anyone tell me what I am doing wrong?</p>
  
  <p><strong>Update:</strong></p>
  
  <p>schema.rb</p>
  
  <pre><code> create_table "scrapbook_entries", force: true do |t|
     t.integer  "scrapbook_id"
     t.integer  "recipe_id"
     t.datetime "created_at"
     t.datetime "updated_at"
     t.integer  "user_id"
   end
  </code></pre>