ReadOnlyFormBuilder

8 Mar, 2008

For RubyOnRails developers, form_for and fields_for are the accepted way of DRYing up form templates. You know the deal; you code 

<% form_for :customer, :url => customers_path() do |customer_form| %>
  <p>
    <label>Name:</label> 
    <%= customer_form.text_field :first_name, :size => 15 %>
    <%= customer_form.text_field :last_name, :size => 20 %>
  </p>
  ... etc ...
<% end %>

and you get 

<form action="/customers" method="post">
  <p>
    <label>Name:</label> 
    <input id="customer_first_name" name="customer[first_name]" size="15" type="text" />
    <input id="customer_last_name" name="customer[last_name]" size="20" type="text" value="" />
  </p>
  ... etc ...
</form>

Rails generates sensible field names and ids for you, and slurps existing values out of the model object. So far, so good.

Lately, I've taken to using the same trick when presenting data, not just when editing it. So, whereas before I might have written: 

  <p>
    <label>Name:</label> 
    <span id="customer_first_name"><%= h @customer.first_name %></span>
    <span id="customer_last_name"><%= h @customer.last_name %></span>
  </p>
  ... etc ...

I'll now code it up as: 

<% fields_for :customer, :builder => ReadOnlyFormBuilder do |customer_form| %>
  <p>
    <label>Name:</label> 
    <%= customer_form.text_field :first_name, :size => 15 %>
    <%= customer_form.text_field :last_name, :size => 20 %>
  </p>
  ... etc ...
<% end %>

and get the same output. (In case you're wondering, the ids are there to help with automated testing).

Note the similarity between the last code snippet and the first one on this page; apart from the first line they're indentical. Usually, I'll put the field-declarations themselves in a partial that's shared between "new", "edit" and "show" actions. That way, your "show" page automatically gets identical layout to the others, just with raw values in place of editable fields.

The ReadOnlyFormBuilder class itself it fairly straightforward - I'm planning to wrap it up into a plugin sometime soon. In the meantime, the implementation of text_field looks something like this: 

def text_field(attribute, options={})
  content_tag("span", html_escape(value_of(attribute)), :id => "#{@object_name}_#{attribute}")
end

def value_of(attribute)
  value = model.send(attribute)
end

def model
  @object || @template.instance_variable_get("@#{@object_name}")
end
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