Drop downs selections for belongs_to in rails

Rails does a lot of work for you out of the box, and with it's form builders you can get it to do some things that make you never want to go back to .NET and databind a control ever again...

Example: Recently I needed to bash together a quick admin screen with a couple of models. We have an organisation model that belongs_to :region and a Region model that has_many :organisations. This dictates a foreign key in organisation that points at a single region. In our generated Rails forms we enter that as a number, but it'd really be a nicer user experience to select the region from a drop down list right?

Enter collection_select. This little helper generates a selection dropdown from a collection (not surprisingly). So if in my organisation_controller's edit method I return @regions = Region.all we can get instant rails gratification.

Finally, since Edit and New use EXACTLY the same form, we're going to try be DRY and pull all the duplicated code out into a partial. I usually use the default form builder to base it on (you know, "form_for(@organisation) do |f|" ) and call "render :partial => f"

When this renders it looks for the partial pointed at by f, which is a form. so _form.html.erb will be rendered. A final note, even though the partial here will be able to see @regions, I like to explicitly pass it in the locals hash, so I know where the heck it came from in 6 months time. If that's confusing you can just strip out that code and use the global set in the organisation controller. Here's the code:

models/organisation.rb

class Organisation < ActiveRecord::Base

  belongs_to :region

end

models/region.rb

class Region< ActiveRecord::Base

  has_many :organisations

end

controllers/organisations_controller.rb

class OrganisationsController < ApplicationController

  # GET /organisations/new

  # GET /organisations/new.xml

  def new

    @organisation = Organisation.new

    @regions = Region.all

    respond_to do |format|

      format.html # new.html.erb

      format.xml  { render :xml => @organisation }

    end

  end

  # GET /organisations/1/edit

  def edit

    @organisation = Organisation.find(params[:id])

    @regions = Region.all

  end
end

views/organisations/_form.html.erb

<%= form.error_messages %>

<p>

  <%= form.label :org_name %><br />

  <%= form.text_field :org_name %>

</p>

<p>

  <%= form.label :org_code %><br />

  <%= form.text_field :org_code %>

</p>

<p>

  <%= form.label :provider %><br />

  <%= form.text_field :provider %>

</p>

<p>

  <%= form.label :region_id %><br />

  <%= collection_select(:organisation, :region_id, regions, :id, :full_name, {:prompt => false}) %>

</p>

views/organisations/edit.html.erb

<% form_for(@organisation) do |f| %>

  #:region => regions exposes the regions array as a local variable in the partial

  <%= render :partial => f, :locals => { :regions => @regions } %>

<p>

  <%= f.submit 'Update' %>

</p>

<% end %>