# 1:1 Relation
class Apple < ActiveRecord::Base
  has_one :banana
end

class Banana < ActiveRecord::Base
  belongs_to :apple
end

# 1:N Relation
class Apple < ActiveRecord::Base
  has_many :bananas
end

class Banana < ActiveRecord::Base
  belongs_to :apple
end

# M:N Relation
class Apple < ActiveRecord::Base
  has_and_belongs_to_many :bananas
end

class Banana < ActiveRecord::Base
  has_and_belongs_to_many :apples
end