Add validation error from associated model to parent

Rails
1

so basically i has this setup (example):

class FellowUser < ActiveRecord::Base
belongs_to :fellow
belongs_to :user
end

class Fellows < ActiveRecord::Base
belongs_to :user
has_many :fellow_users
has_many :fellows, :through => :fellow_users
end

what i want, in rspec terms, is this:

i want validation to fire if i associate user twice to fellow

it “should not be able to be fellow twice” do
@fellow.fellows << users(:george)
@fellow.fellows << users(:quentin)
@ft.fellows.size.should == 2

# same user second time
@fellow.fellows << users(:george)


# tried @fellow.should_not be_valid - not working either
@fellow.should have(1).error_on(:fellows)

@fellow.fellows.size.should == 2

end

in join model:

i tried this with no luck:
validates_uniqueness_of :fellow_id, :scope => :user_id

i tried this with same result:
validate do |record|
# add_to_base, can be add(:fellows, --//–) and no luck here
record.errors.add_to_base(“duplicating hitcher”) if
record.fellow.fellows.size!=record.fellow.fellows.uniq.size
end

in parent model:

this makes model not valid but it still saves and record is add to

join model

   validate do |record|
     record.errors.add(:fellows, "you can't be fellow twice") if

record.fellows.size!=record.fellows.uniq.size
end

Any suggestions will be greatly appreciated.