CertificationDomain.create :domain => Domain.first, :certification => Certification.first, :ratio => "10"
=> true

certifications_domains.yml

1:
  domain: "#politique_et_gestion_du_programme_mca"
  certification: "#business-continuity-institute"
  ratio: 10

2:
  domain: "#comprendre_l_organisme"
  certification: "#business-continuity-institute"
  ratio: 25

class CertificationDomain < ActiveRecord::Base
  set_table_name :certifications_domains
  # relations
  belongs_to :certification
  belongs_to :domain

  # validations
  validates_presence_of :ratio
  validates_numericality_of :ratio, :only_integer => true, :less_than_or_equal_to => 100, :greater_than_or_equal_to => 0
end

rake db:fixtures:load

Mysql2::Error: Unknown column 'domain' in 'field list': INSERT INTO `certifications_domains` (`domain`, `certification`, `ratio`) VALUES ('#politique_et_gestion_du_programme_mca', '#business-continuity-institute', 10)