rails_guides/nested_model_forms.html at gh-pages · rubykr/rails_guides · GitHub

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      <h2>Rails nested model forms</h2>
<p>Creating a form for a model <em>and</em> its associations can become quite tedious. Therefore Rails provides helpers to assist in dealing with the complexities of generating these forms <em>and</em> the required <span class="caps">CRUD</span> operations to create, update, and destroy associations.</p>
<p>In this guide you will:</p>
<ul>
	<li>do stuff</li>
</ul>

            <div id="subCol">
        <h3 class="chapter"><img src="images/chapters_icon.gif" alt="" />Chapters</h3>
        <ol class="chapters">
<li><a href="#model-setup">Model setup</a><ul><li><a href="#activerecord-base-model">ActiveRecord::Base model</a></li> <li><a href="#custom-model">Custom model</a></li></ul></li><li><a href="#views">Views</a><ul><li><a href="#controller-code">Controller code</a></li> <li><a href="#form-code">Form code</a></li></ul></li></ol></div>
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        <div class='note'><p>This guide assumes the user knows how to use the <a href="form_helpers.html">Rails form helpers</a> in general. Also, it’s <strong>not</strong> an <span class="caps">API</span> reference. For a complete reference please visit <a href="http://api.rubyonrails.org/">the Rails <span class="caps">API</span> documentation</a>.</p></div>
<h3 id="model-setup">1 Model setup</h3>
<p>To be able to use the nested model functionality in your forms, the model will need to support some basic operations.</p>
<p>First of all, it needs to define a writer method for the attribute that corresponds to the association you are building a nested model form for. The <tt>fields_for</tt> form helper will look for this method to decide whether or not a nested model form should be build.</p>
<p>If the associated object is an array a form builder will be yielded for each object, else only a single form builder will be yielded.</p>
<p>Consider a Person model with an associated Address. When asked to yield a nested FormBuilder for the <tt>:address</tt> attribute, the <tt>fields_for</tt> form helper will look for a method on the Person instance named <tt>address_attributes=</tt>.</p>
<h4 id="activerecord-base-model">1.1 ActiveRecord::Base model</h4>
<p>For an ActiveRecord::Base model and association this writer method is commonly defined with the <tt>accepts_nested_attributes_for</tt> class method:</p>
<h5 id="has_one">1.1.1 has_one</h5>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class Person &lt; ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address
end
</pre>
</div>
</notextile>

<h5 id="belongs_to">1.1.2 belongs_to</h5>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class Person &lt; ActiveRecord::Base
  belongs_to :firm
  accepts_nested_attributes_for :firm
end
</pre>
</div>
</notextile>

<h5 id="has_many-has_and_belongs_to_many">1.1.3 has_many / has_and_belongs_to_many</h5>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class Person &lt; ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects
end
</pre>
</div>
</notextile>

<h4 id="custom-model">1.2 Custom model</h4>
<p>As you might have inflected from this explanation, you <em>don’t</em> necessarily need an ActiveRecord::Base model to use this functionality. The following examples are sufficient to enable the nested model form behaviour:</p>
<h5 id="single-associated-object">1.2.1 Single associated object</h5>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class Person
  def address
    Address.new
  end

  def address_attributes=(attributes)
    # ...
  end
end
</pre>
</div>
</notextile>

<h5 id="association-collection">1.2.2 Association collection</h5>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class Person
  def projects
    [Project.new, Project.new]
  end

  def projects_attributes=(attributes)
    # ...
  end
end
</pre>
</div>
</notextile>

<div class='note'><p>See (<span class="caps">TODO</span>) in the advanced section for more information on how to deal with the <span class="caps">CRUD</span> operations in your custom model.</p></div>
<h3 id="views">2 Views</h3>
<h4 id="controller-code">2.1 Controller code</h4>
<p>A nested model form will <em>only</em> be build if the associated object(s) exist. This means that for a new model instance you would probably want to build the associated object(s) first.</p>
<p>Consider the following typical RESTful controller which will prepare a new Person instance and its <tt>address</tt> and <tt>projects</tt> associations before rendering the <tt>new</tt> template:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class PeopleController &lt; ActionController:Base
  def new
    @person = Person.new
    @person.built_address
    2.times { @person.projects.build }
  end

  def create
    @person = Person.new(params[:person])
    if @person.save
      # ...
    end
  end
end
</pre>
</div>
</notextile>

<div class='note'><p>Obviously the instantiation of the associated object(s) can become tedious and not <span class="caps">DRY</span>, so you might want to move that into the model itself. ActiveRecord::Base provides an <tt>after_initialize</tt> callback which is a good way to refactor this.</p></div>
<h4 id="form-code">2.2 Form code</h4>
<p>Now that you have a model instance, with the appropriate methods and associated object(s), you can start building the nested model form.</p>
<h5 id="standard-form">2.2.1 Standard form</h5>
<p>Start out with a regular RESTful form:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
&lt;%= form_for @person do |f| %&gt;
  &lt;%= f.text_field :name %&gt;
&lt;% end %&gt;
</pre>
</div>
</notextile>

<p>This will generate the following html:</p>
<notextile>
<div class="code_container">
<pre class="brush: xml; gutter: false; toolbar: false">
&lt;form action=&quot;/people&quot; class=&quot;new_person&quot; id=&quot;new_person&quot; method=&quot;post&quot;&gt;
  &lt;input id=&quot;person_name&quot; name=&quot;person[name]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;
&lt;/form&gt;
</pre>
</div>
</notextile>

<h5 id="nested-form-for-a-single-associated-object">2.2.2 Nested form for a single associated object</h5>
<p>Now add a nested form for the <tt>address</tt> association:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
&lt;%= form_for @person do |f| %&gt;
  &lt;%= f.text_field :name %&gt;

  &lt;%= f.fields_for :address do |af| %&gt;
    &lt;%= f.text_field :street %&gt;
  &lt;% end %&gt;
&lt;% end %&gt;
</pre>
</div>
</notextile>

<p>This generates:</p>
<notextile>
<div class="code_container">
<pre class="brush: xml; gutter: false; toolbar: false">
&lt;form action=&quot;/people&quot; class=&quot;new_person&quot; id=&quot;new_person&quot; method=&quot;post&quot;&gt;
  &lt;input id=&quot;person_name&quot; name=&quot;person[name]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;

  &lt;input id=&quot;person_address_attributes_street&quot; name=&quot;person[address_attributes][street]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;
&lt;/form&gt;
</pre>
</div>
</notextile>

<p>Notice that <tt>fields_for</tt> recognized the <tt>address</tt> as an association for which a nested model form should be build by the way it has namespaced the <tt>name</tt> attribute.</p>
<p>When this form is posted the Rails parameter parser will construct a hash like the following:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
{
  &quot;person&quot; =&gt; {
    &quot;name&quot; =&gt; &quot;Eloy Duran&quot;,
    &quot;address_attributes&quot; =&gt; {
      &quot;street&quot; =&gt; &quot;Nieuwe Prinsengracht&quot;
    }
  }
}
</pre>
</div>
</notextile>

<p>That’s it. The controller will simply pass this hash on to the model from the <tt>create</tt> action. The model will then handle building the <tt>address</tt> association for you and automatically save it when the parent (<tt>person</tt>) is saved.</p>
<h5 id="nested-form-for-a-collection-of-associated-objects">2.2.3 Nested form for a collection of associated objects</h5>
<p>The form code for an association collection is pretty similar to that of a single associated object:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
&lt;%= form_for @person do |f| %&gt;
  &lt;%= f.text_field :name %&gt;

  &lt;%= f.fields_for :projects do |pf| %&gt;
    &lt;%= f.text_field :name %&gt;
  &lt;% end %&gt;
&lt;% end %&gt;
</pre>
</div>
</notextile>

<p>Which generates:</p>
<notextile>
<div class="code_container">
<pre class="brush: xml; gutter: false; toolbar: false">
&lt;form action=&quot;/people&quot; class=&quot;new_person&quot; id=&quot;new_person&quot; method=&quot;post&quot;&gt;
  &lt;input id=&quot;person_name&quot; name=&quot;person[name]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;

  &lt;input id=&quot;person_projects_attributes_0_name&quot; name=&quot;person[projects_attributes][0][name]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;
  &lt;input id=&quot;person_projects_attributes_1_name&quot; name=&quot;person[projects_attributes][1][name]&quot; size=&quot;30&quot; type=&quot;text&quot; /&gt;
&lt;/form&gt;
</pre>
</div>
</notextile>

<p>As you can see it has generated 2 <tt>project name</tt> inputs, one for each new <tt>project</tt> that’s build in the controllers <tt>new</tt> action. Only this time the <tt>name</tt> attribute of the input contains a digit as an extra namespace. This will be parsed by the Rails parameter parser as:</p>
<notextile>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
{
  &quot;person&quot; =&gt; {
    &quot;name&quot; =&gt; &quot;Eloy Duran&quot;,
    &quot;projects_attributes&quot; =&gt; {
      &quot;0&quot; =&gt; { &quot;name&quot; =&gt; &quot;Project 1&quot; },
      &quot;1&quot; =&gt; { &quot;name&quot; =&gt; &quot;Project 2&quot; }
    }
  }
}
</pre>
</div>
</notextile>

<p>You can basically see the <tt>projects_attributes</tt> hash as an array of attribute hashes. One for each model instance.</p>
<div class='note'><p>The reason that <tt>fields_for</tt> constructed a form which would result in a hash instead of an array is that it won&#8217;t work for any forms nested deeper than one level deep.</p></div>
<div class='info'><p>You <em>can</em> however pass an array to the writer method generated by <tt>accepts_nested_attributes_for</tt> if you&#8217;re using plain Ruby or some other <span class="caps">API</span> access. See (<span class="caps">TODO</span>) for more info and example.</p></div>

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