title: "Couldn't find <object> without an ID"
body: <p>I'm having problems implementing a kind of comments form, where comments (called "microposts") belong_to both users and posts, users have_many comments, and posts (called "propositions") have_many comments.</p>
  
  <p>My code for the comments form is:</p>
  
  <pre><code>&lt;%= form_for @micropost do |f| %&gt;
    &lt;div class="field"&gt;
        &lt;%= f.text_area :content %&gt;
    &lt;/div&gt;
  
    &lt;div class="actions"&gt;
      &lt;%= f.submit "Submit" %&gt;
    &lt;/div&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>The MicropostsController has this in the create action:</p>
  
  <pre><code>  def create
  
        @proposition = Proposition.find(params[:proposition_id])
  
    @micropost = current_user.microposts.build(params[:micropost])
    @micropost.proposition = @proposition
    if @micropost.save
        flash[:success] = "Contribution submitted"
        redirect_to root_path
    else
            @feed_items = []    
        render 'pages/home'
    end
    end
  </code></pre>
  
  <p>The form for creating a new micropost is on the same page as a proposition, yet the proposition id doesn't seem to get passed at any point.</p>
  
  <p>This is the error I get on submitting the micropost form:</p>
  
  <blockquote>
    <p>ActiveRecord::RecordNotFound in
    MicropostsController#create</p>
    
    <p>Couldn't find Proposition without an
    ID</p>
  </blockquote>
  
  <p>Parameters are:</p>
  
  <blockquote>
    <p>{"commit"=>"Submit", 
    "micropost"=>{"proposition_id"=>"", 
    "content"=>"First comment"}, 
    "authenticity_token"=>"TD6kZaHv3CPWM7xLzibEbaLJHI0Uw43H+pq88HLZFjc=",
    "utf8"=>"âœ“"}</p>
  </blockquote>
  
  <p>I'm completely new to rails and very new to coding anything at all, so I'd be grateful for any help you can give me!</p>
  
  <p>Thanks in advance.</p>