Symmetry
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Symmetry gives you a simple way of creating symmetric has_and_belongs_to_many relationships (friends, neighbors, etc.) in your Active Record models.
Installation
Add it to your Gemfile:
gem "symmetry"
Install the migrations:
rake symmetry_engine:install:migrations db:migrate
Usage
Define a symmetric relationship:
class User < ActiveRecord::Base
symmetric_relationship :friends
end
class Country < ActiveRecord::Base
symmetric_relationship :neighbors, polymorphic_relationship_name: :neighborships
end
Use it:
# User
irb(main)> u = User.create(name: "Daniel")
=> #<User id: 1, name: "Daniel", ...>
irb(main)> u.friends
=> []
irb(main)> u.friend_relationships
=> []
# Country
irb(main)> c = Country.create(name: "Sweden")
=> #<Country id: 1, name: "Sweden", ...>
irb(main)> c.neighbors << Country.create(name: "Norway")
=> [#<Country id: 2, name: "Norway", ...>]
irb(main)> c.neighbors
=> [#<Country id: 2, name: "Norway", ...>]
irb(main)> Country.find(2).neighbors
=> [#<Country id: 1, name: "Sweden", ...>]
irb(main)> c.neighborships
=> [#<SymmetricRelationship id: 1, owner_id: 1, owner_type: "Country", relation_id: 2, relation_type: "Country", relationship_name: "neighbors" ...>, ...]
It is possible to define multiple symmetric relationships on the same model:
class Politician
symmetric_relationship :friends
symmetric_relationship :enemies
end
irb(main)> p1 = Politician.create(name: 'A')
=> #<Politician id: 1, name: "A", ...>
irb(main)> p2 = Politician.create(name: 'B')
=> #<Politician id: 2, name: "B", ...>
irb(main)> p3 = Politician.create(name: 'C')
=> #<Politician id: 3, name: "C", ...>
irb(main)> p1.friends
=> []
irb(main)> p1.enemies
=> []
irb(main)> p1.friends << p2
=> [#<Politician id: 2, name: "B", ...>]
irb(main)> p1.enemies << p3
=> [#<Politician id: 3, name: "C", ...>]
irb(main)> p1.friends.count
=> 1
irb(main)> p1.enemies.count
=> 1
irb(main)> p2.friends.count
=> 1
irb(main)> p2.enemies.count
=> 0