问题
I found this recently when trying to do bidirectional relationships in rails (http://www.dweebd.com/sql/modeling-bidirectional-graph-edges-in-rails/)
class Befriending < ActiveRecord::Base
belongs_to :initiator, :class_name => :User
belongs_to :recipient, :class_name => :User
after_create do |b|
BefriendingEdge.create!(:user => b.initiator, :befriending => b)
BefriendingEdge.create!(:user => b.recipient, :befriending => b)
end
end
class BefriendingEdge < ActiveRecord::Base
belongs_to :user
belongs_to :befriending
end
class User < ActiveRecord::Base
has_many :befriending_edges
has_many :friends, :through => :befriending_edges, :source => :user
has_many :befriendings, :through => :befriending_edges, :source => :befriending
end
But I just don't quite understand how it works. Can anyone helps explain to me. It looks like a double belongs_to. Just not quite understanding this.
Thanks
回答1:
- I'm a user
- I have friends
- My friends are also users
The way to model this using a graph (http://en.wikipedia.org/wiki/Graph_%28mathematics%29) is with
- nodes that represent users/friends
- edges that represent friendship links
So yes: in databases terms, "users belong to users" : my friends are also users. But in addition, friendship is bi-directional: if we're friends that means, I'm your friend AND you're my friend.
Also, using a separate model to store edges/relationships allows you potentially store additional information about the friendship (e.g. "friends since").
来源:https://stackoverflow.com/questions/836800/graph-edges-rails