model scenario:
A node can belong to a parent node and can have child nodes.
models/node.rb
class Node < ActiveRecord::Base
has_many :children, class_name: "Node", foreign_key: "parent_id"
belongs_to :parent, class_name: "Node"
end
db/migrations/20131031144907_create_nodes.rb
class CreateNodes < ActiveRecord::Migration
def change
create_table :nodes do |t|
t.timestamps
end
end
end
And then I want to do I migration to add the relations:
class AddNodesToNodes < ActiveRecord::Migration
def change
add_column :nodes, :parent_id, :integer
# how do i add childen?
end
end
How do i add the has_many relationship in the migratation?
You've done everything you needed to do.You can find the more informations in this page:
Source: http://guides.rubyonrails.org/association_basics.html
node.parent
will find the parent_id
is the node id and return the parent.
node.children
will find the parent_id
is the node id and return the children.
And when you add relations, you can do like this in Rails 4:
## rails g migration AddNodesToNodes parent:belongs_to
class AddNodesToNodes < ActiveRecord::Migration
def change
add_reference :nodes, :parent, index: true
end
end
Per RailsGuides, this is an example of a Self-Join.
# model
class Node < ActiveRecord::Base
has_many :children, class_name: "Node", foreign_key: "parent_id"
belongs_to :parent, class_name: "Node"
end
# migration
class CreateNodes < ActiveRecord::Migration
def change
create_table :nodes do |t|
t.references :parent, index: true
t.timestamps null: false
end
end
end
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