params[:fu] #4 ) Use the magical <association_name>_ids=([...array of ids]) association proxy method.

In the last blog entry, we looked at an example of having to build a new parent model and build its join models all in two lines of code. But if you are trying to just assign a list of has_many or has_and_belongs_to_many models, you don't have to use the fields_for and index trick. You can just use an association proxy method. Let's check it out:

class Reader < ActiveRecord::Base
  has_and_belongs_to_many :blogs
end

<% form_for @reader do |f| %>
  <%= f.text_field :name %>

  <%= f.collection_select :blog_ids, Blog.find(:all), :id, :name, {}, :multiple => true %>

  <%= f.submit 'Create' %>
<% end %>

generates:

<select id="reader_blog_ids" multiple="multiple" name="reader[blog_ids][]">
  <option value="1">Martin Fowler</option>
  <option value="2">Stephen Chu</option>
</select> 

Processing ReadersController#create (for 127.0.0.1 at 2008-01-14 21:12:56) [POST]
Parameters: { "commit"             => "Create",
              "reader"             => { "name" => "stephen chu"
                                        "blog_ids" => ["1", "2"] },
              "authenticity_token" => "238ba79b8282882ba01d840352616c2cc79280f0",
              "action"             => "create",
              "controller"         => "readers" }

And now, in my controller action, the creation of the reader only takes one line of code:

def create
  @reader = Reader.new(params[:reader])

... thanks to the following has_many (or habtm) association assignment method:

@reader.blog_ids = [...many ids...]

So, stop doing any more params-munging code in your controller action like these:

def create
  ...
  params[:blog_ids].each do |id|
    @reader.blogs << Blog.find(id)
  end
end

Get on the Rails bandwagon!