 <p>Use <code>fields_for</code> for the associated models.<br> <strong>There should be no square brackets arround the parameters of <code>fields_for</code></strong></p> <p>In your code example, I cannot find the relation between <code>Patient</code> and <code>Diagnosis</code>, and the plural of diagnosis is <strong>diagnoses</strong>, you can specify this in <code>config/initializers/inflections.rb</code>:</p> <pre><code>ActiveSupport::Inflector.inflections do |inflect| inflect.irregular 'diagnosis','diagnoses' end </code></pre> <p>So your <code>Patient</code> model should contain</p> <pre><code>class Patient &lt; ActiveRecord::Base attr_accessible :age, :name, :city, :street, :number has_many :diagnoses end </code></pre> <p>And you can write in your form:</p> <pre><code> &lt;div class="field"&gt; &lt;%= f.label :content %&gt;&lt;br /&gt; &lt;%= f.text_field :content %&gt; &lt;/div&gt; &lt;%= fields_for(@patient, @patient.diagnoses.build) do |u| %&gt; &lt;div class="field"&gt; &lt;%= u.label :content %&gt;&lt;br /&gt; &lt;%= u.text_field :content %&gt; &lt;/div&gt; &lt;% end %&gt; &lt;div class="actions"&gt; &lt;%= f.submit %&gt; &lt;/div&gt; </code></pre>