在同一模型中创建has_one关联

Question

Let us consider the below model.

User{ id:number(primary_key), column1:string, column2:string)

Now, the column1 can be any string, but column2 can be either null, or have values from column1. However column1 and column2 can't be same. IE column2 will be the foreign key and it will reference column1.

How will i create a has_one relationship with these constraints.

Something like,

has_one :master_agreement, :class_name => 'User', :foreign_key => 'column2', :primary_key => 'column1'

but the above mentioned doesn't work for me.

Answer 1

To setup a self-joining relationship in rails you want to use belongs_to not has_one as the key difference is that belongs_to places the foreign key column on the model's table - while has_one places it on the other end.

class User < ActiveRecord::Base
  belongs_to :company # references users.company_id
  has_one :company # references companies.user_id
end

Lets consider a system where you have a hierarchy of categories:

class Category < ActiveRecord::Base
  belongs_to :parent, class_name: 'Category'
  has_many :subcategories, class_name: 'Category', foreign_key: 'parent_id'
end 

This creates a self joining relationship with the categories.parent_id foreign key column which references categories.id .

While you could potentially use something different than categories.id as the reference you are just making things harder on yourself as the id column already is an indexed primary key column.