title: Rails if @model.save is throwing an error
body: <p>I have a Message model. Whenever I run <code>if @message.save</code> in my controller I get this error:</p>
  
  <pre><code>ERROR:  null value in column "conversation_id" violates not-null constraint
  </code></pre>
  
  <p>I just want to redirect if save fails.</p>
  
  <pre><code>  def create
      @message = Message.new(message_params)
      if @message.save
        # redirect
      else
        # render new
      end
    end 
  </code></pre>
  
  <p>My schema for messages</p>
  
  <pre><code>  create_table "messages", force: true do |t|
      t.string   "body",                                    null: false
      t.integer  "conversation_id",                         null: false
      t.datetime "created_at"
      t.datetime "updated_at"
    end
  </code></pre>