Validation association not working

Question

I have two models: Location and User. Location has_many :users and User belongs_to :location. Now I have a form that uses something similar to:

 <%= form_for @user do |f| %>
 ...
   <%= f.select(:location,:location_id,Location.all.collect{|location| location.name}) %>
 ...
 <%end%>

When inspected it appears that it generates the correct form field for it:

 <select id="user_location_id" name="user[location_id]">
    <option value="Clearfield">Clearfield</option>
    <option value="San Diego">San Diego</option>
  </select>

According to Ruby on Rails: question about validates_presence_of validates_presence_of should work on the model name and in my models I have:

User.rb:

  class User < ActiveRecord::Base
       belongs_to :location
       validates_presence_of :location
   ...

Location.rb:

 class Location < ActiveRecord::Base
      has_many :users    
 ...

What am I doing wrong that would make it so that validates_presence_of doesn't see location when I use validates_presence_of :location, but for some reason works when I use location_id?

Answer 1

validates_presence_of :location doesn't add error in following situation:

user = User.new(:location => Location.new)
user.valid? # => true

On the other hand, validates_presence_of :location_id doesn't add error if location with given id doesn't exist:

user = User.new(:location_id => 123)
user.valid? #=> true
Location.find_by_id(user.location_id) #=> nil