Question

I want to create an app that uses both MongoDB and MySQL. Specifically, I want mongodb to store all the users’ comments while MySQL will store the User model.

class User < ActiveRecord::Base
  has_many :comments
end

class Comment
  include Mongoid::Document
  include Mongoid::Timestamps
  belongs_to :user
end

well, everything looks good except when I go to the rails console and run this.

k = Comment.new
k.user = User.first

I got

NoMethodError: User Load (0.3ms) SELECT users.* FROM users
WHERE users._id = 1 Mysql2::Error: Unknown column ‘users._id’ in
‘where clause’: SELECT users.* FROM users WHERE users._id = 1
undefined method `from_map_or_db’ for

It looks like that the := method is looking for the _id of the model instea of the id? Is there a workaround to get this working automatically or do I need to create my own = method?
Has anyone tried the same configuration before? If so, what are the steps to get all these to work?

Answer 1

This is not gonna work like you want it to. Your belongs_to :user in Comment is telling Mongoid to make this association in MongoDB; in order to make ActiveRecord associations, your class must inherit from ActiveRecord::Base or include ActiveRecord::Model–and you can’t do both!

Probably the best way to do this–and I don’t know how difficult it would be–is to write your own methods to associate the Users and Comments together.