instance method grouped_collection_select

Returns <select>, <optgroup> and <option> tags for the collection of existing return values of method for object‘s class. The value returned from calling method on the instance object will be selected. If calling method returns nil, no selection is made without including :prompt or :include_blank in the options hash.

Parameters:

• object - The instance of the class to be used for the select tag

• method - The attribute of object corresponding to the select tag

• collection - An array of objects representing the <optgroup> tags.

• group_method - The name of a method which, when called on a member of collection, returns an array of child objects representing the <option> tags. It can also be any object that responds to call, such as a proc, that will be called for each member of the collection to retrieve the value.

• group_label_method - The name of a method which, when called on a member of collection, returns a string to be used as the label attribute for its <optgroup> tag. It can also be any object that responds to call, such as a proc, that will be called for each member of the collection to retrieve the label.

• option_key_method - The name of a method which, when called on a child object of a member of collection, returns a value to be used as the value attribute for its <option> tag.

• option_value_method - The name of a method which, when called on a child object of a member of collection, returns a value to be used as the contents of its <option> tag.

Example object structure for use with this method:

# attributes: id, name
class Continent < ActiveRecord::Base
  has_many :countries
end

# attributes: id, name, continent_id
class Country < ActiveRecord::Base
  belongs_to :continent
end

# attributes: id, name, country_id
class City < ActiveRecord::Base
  belongs_to :country
end

Sample usage:

grouped_collection_select(:city, :country_id, @continents, :countries, :name, :id, :name)

Possible output:

<select name="city[country_id]" id="city_country_id">
  <optgroup label="Africa">
    <option value="1">South Africa</option>
    <option value="3">Somalia</option>
  </optgroup>
  <optgroup label="Europe">
    <option value="7" selected="selected">Denmark</option>
    <option value="2">Ireland</option>
  </optgroup>
</select>