Rails ActiveRecord：相同模型之间的两个has_one关系

Question

I have a model, Project, that has 2 associations to the same model:

belongs_to :source_connection, class: Connection
belongs_to :destination_connection, class: Connection

That works fine - I can access connection methods through project without any issues.

I'm a little confused on how to do the inverse, however. I started with a rather optimistic:

has_one :project

on the Connection model, and unsurprisingly, it throws an

ActiveModel::MissingAttributeError: can't write unknown attribute 'connection_id'

error when I try to access the project from the connection itself.

If anyone knows how I can declare the association on the connection side, I'd be appreciative. Cheers.

Answer 1

Associations

You will probably be best looking at the foreign_key arguments for ActiveRecord Associations:

#app/models/project.rb
Class Project < ActiveRecord::Base
   belongs_to :source_connection, class: "Connection", foreign_key: "source_connection_id"
   belongs_to :destination_connection, class: "Connection", foreign_key: "destination_connection_id"
end

#app/models/connection.rb
Class Connection < ActiveRecord::Base
   has_many :projects, foreign_key: "source_connection_id"
end

The issue you have is that since you're not using the foreign_key option in your associations, Rails will be looking for the standard foreign_key associations inside your schema (typically model_name_id ).

--

Error

can't write unknown attribute 'connection_id'

I don't know why it's complaining about writing , but the reason is likely that you don't have the correct foreign key set up for your association. Typically, Rails will be looking for the model_name_id - but since you're not, you'll need to set the relative key in your models (as demonstrated)

Answer 2

has_one :project, foreign_key: 'source_connection_id'

Rails is looking for connection_id as the error says, as you've used source_connection and destination_connection you need to tell it to use the correct foreign key to look it up.

You might want to define them as:

has_one :source_project, foreign_key: 'source_connection_id'
has_one :destination_project, foreign_key: 'destination_connection_id'

Because you won't be able to call them both project.