Rails - column not found for defined 'has_many' relationship - ruby-on-rails

Question

I define a Post class which can link or be linked to multiple posts. To do this I added a PostLink class which specifies post_to and post_from.
I generated the PostLink class by rails g model post_link from_post:integer to_post:integer and of course rake db:migrate, and added
belongs_to :from_post, :class_name => 'Post'
belongs_to :to_post, :class_name => 'Post'
to the class.
And I also have has_many :post_links in my Post class.
I ran rails console and Post.new.post_links and got nil printed out, which is expected. However after I save a Post using
p = Post.new
p.save
and then run p.post_links, it prints out the following error message:
SQLite3::SQLException: no such column: post_links.post_id: SELECT "post_links".*
FROM "post_links" WHERE "post_links"."post_id" = 1
So anybody know why after saving it to the database post_link can not be accessed?

Answer 1

The has_many :post_links association in Post throws an error because it assumes the foreign key in post_links is post_id by default. Since you are using from_post_id and to_post_id, you will have to find a way to group the post_links for "from" posts and "to" posts to get the total set of post_links for a post.
One approach could be to define two associations on Post and an additional method to add the sets together:
class Post < ActiveRecord::Base
has_many :from_post_links, :class_name => 'PostLink', :foreign_key => :from_post_id
has_many :to_post_links, :class_name => 'PostLink', :foreign_key => :to_post_id'
def post_links
from_post_links + to_post_links
end
end
As another option, you could provide special sql to the association to group the sets in a single query:
class Post < ActiveRecord::Base
has_many :post_links, :finder_sql => Proc.new {
%Q{
SELECT *
FROM post_links pl
WHERE pl.to_post_id = #{id}
OR pl.from_post_id = #{id}
}
}