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If you are using Rails' form helper collection_check_boxes, it's not entirely obvious how you might go about customizing the label's html.

The "Normal Way"

...or what frequently shows up in the documentation.

Say we have the following Ruby classes:

class Post < ActiveRecord::Base
  has_and_belongs_to_many :authors
end
class Author < ActiveRecord::Base
  has_and_belongs_to_many :posts
  def name_with_initial
    "#{first_name.first}. #{last_name}"
  end
end

The easiest way to create a group of checkboxes is with collection_check_boxes:

<!-- app/views/carts/new.html.erb -->
<%= collection_check_boxes(:post, :author_ids, Author.all, :id, :name_with_initial) %>

This generates the following HTML:

<input id="post_author_ids_1" name="post[author_ids][]" type="checkbox" value="1" checked="checked" />
<label for="post_author_ids_1">D. Heinemeier Hansson</label>
<input id="post_author_ids_2" name="post[author_ids][]" type="checkbox" value="2" />
<label for="post_author_ids_2">D. Thomas</label>
<input id="post_author_ids_3" name="post[author_ids][]" type="checkbox" value="3" />
<label for="post_author_ids_3">M. Clark</label>
<input name="post[author_ids][]" type="hidden" value="" />

The Not-so-normal Way

If you don't have access to the Author class, as was my case when working with the Stripe API, you have two options.

Add to the open ruby class

Opening a class definition on the class for which you don't have access allows you to define new methods on a class:

class Author
  def name_with_initial
    # code accessing instance
  end
end

...or

Pass a block to the form helper

collection_check_boxes(:post, :author_ids, Author.all, :id, :name_with_initial) do |b|
  b.check_box
  b.label do
    b.object.first_name.first + ". " + b.object.last_name
  end
end

Boom! The instance for each of the radio buttons is available on the object method as seen above.

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