models.rb

class TariffSchemeFeeRule < ActiveRecord::Base
  belongs_to :tariff_scheme
  belongs_to :fee_rule
  belongs_to :currency

  validates :currency_id, uniqueness: { scope: [:fee_rule_id, :tariff_scheme_id] }, presence: true
end

class TariffScheme < ActiveRecord::Base
  accepts_nested_attributes_for :tariff_scheme_fee_rules, allow_destroy: true
end