person.rb

class Person < ActiveRecord::Base
  belongs_to :country

  scope :minor, proc { where('birthday >= ?', 18.years.ago) }
  scope :adult, proc { where('birthday <= ?', 18.years.ago) }

  scope :male, where(gender: 'male')
  scope :female, where(gender: 'female')

  def self.living_in(country)
    joins(:country).where('countries.name = ?', country)
  end
end

people = Person.minor.male.living_in('Spain')

people.order(:birthday).each do |person|
  # ...
end