Ruby on Rails

Hi!

I have the following classes  for  postgres rails 4 project :

class Medhospital < ActiveRecord::Base
  belongs_to :pcp
  belongs_to :hospital
  validates_uniqueness_of :pcp_id, :scope => :hospital_id
end

class Pcp < ActiveRecord::Base
    has_many :scheds
    has_many :medhospitals, dependent: :destroy
    has_many :hospitals, through: :medhospitals
    has_many :patients, through: :scheds
    accepts_nested_attributes_for :medhospitals, reject_if: :all_blank, allow_destroy: true
end

class Hospital < ActiveRecord::Base
    has_many :scheds
    has_many :medhospitals
    has_many :pcps, through: :medhospitals
end

I am forcing the error to test and the validation for the uniqueness it is not working instead I get this exception:

PG::UniqueViolation - ERROR:  duplicate key value violates unique constraint "index_medhospitals_on_pcp_id_and_hospital_id"

BTW I am testing manually  from a view form.

Am I doing something wrong ? 

Thanks

-Luis