problem.rb

# The classes
class ReducaoZ < ActiveRecord::Base
  self.table_name = 'reducaoz'

  belongs_to :empresa
  belongs_to :impressorafiscal, foreign_key: 'impressorafiscal_id', class_name: 'ImpressoraFiscal'
  belongs_to :usuario_cadastro, foreign_key: 'usuariocadastro_id', class_name: 'Usuario'
  belongs_to :usuario_alteracao, foreign_key: 'usuarioalteracao_id', class_name: 'Usuario'

  has_many :aliquotas, primary_key: 'id', foreign_key: 'reducaoz_id', class_name: 'Aliquota', dependent: :delete_all
end


class Aliquota < ActiveRecord::Base
  self.table_name = 'aliquota'
  
  belongs_to :reducaoz, class_name: 'ReducaoZ', primary_key: 'reducaoz_id', foreign_key: 'id'
end


# The method
json_data.aliquotas_list.each_with_index do |al, index|
        aliquota = reducao.aliquotas.build
        # populate aliquota
        red.aliquotas << aliquota
      end
red.save!