Validating the presence of foreign keys /associated objects

hi guys,

I been trying to find out on how to validate the presence of foreign
keys/associated objects and the threads that talk about this are
rather dated and unanswered. I looked at

1)
http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/f8175db2a8321750/d565496a3672f19f?lnk=gst&q=validates+presence+foreign#d565496a3672f19f
2)
http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/cff66eead0e5580b/c032b19f1832b2e0?lnk=gst&q=validates+presence+foreign#c032b19f1832b2e0

Consider this:

class Part < ActiveRecord::Base
  belongs_to :brand
  belongs_to :category
  belongs_to :sub_category
  belongs_to :user
  belongs_to :status
  has_many :fits
  has_many :images, :dependent => :destroy

validates_presence_of :title, :description, :category, :location, :contact_name, :contact_number, :condition
end

class Brand < ActiveRecord::Base
  has_many :designs
  validates_presence_of :name
end

class Fit < ActiveRecord::Base
    belongs_to :Part
    belongs_to :Brand

    validates_presence_of :part_id

end

When I hit the "submit" button to purposely try and create a new
"Fit" object without selected a Brand (the brand is a drop down box)
and the Part ID is a hidden field, no errors come up.

How do I validate the presence of the "Part ID" (or part object)
within the Fit model?
I tried putting " validates_presence_of :part_id" and it does not
work.

Any ideas, guys?

thanks

class Fit < ActiveRecord::Base
belongs_to :Part
belongs_to :Brand

It probably doesn't make a difference, but conventionally the
attribute ids should be lowercase

     belongs_to :part
     belongs_to :brand

validates_presence_of :part_id

The association name is :part not :part_id
so

    validates_presence_of :part

HTH