My guess is what you really want is for lending to be a many-to-many mapping of users to equipments.
class User < ActiveRecord::Base
has_many :lendings
has_many :equipments, :through => :lendings
# etc
end
class Equipment < ActiveRecord::Base
has_many :lendings
has_many :users, :through => :lendings
# etc
end
class Lending < ActiveRecord::Base
belongs_to :user
belongs_to :equipment
# etc
end
You can write methods to give the "current" user for an equipment via the latest active lending, and vice-versa, plus you have a convenient history of all lendings of an item of equipment.