Association between mongo and postgres model

Question

I have two database in one app

Postgres
MongoDB

Except one model, all models are in Postgres Model name Message is in MongoDB as following

message.rb - MongoDB

class Message
  include Mongoid::Document

  field :content,         type: String
  field :user_id,         type: Integer
end

user.rb - Postgres
```
class User < ApplicationRecord
end
```

Now I want to associate user model with message model, to perform eagerloading.

If I write belongs_to :user, and then perform Message.last.user then following error will raise

NoMethodError (undefined method `_association' for []:Array)

Is there anyway to fix this issue?

mu is too short · Accepted Answer · 2018-04-11 05:39:55Z

3

You can't really make cross-database associations like that. But, depending on how much of the association you need, it might be easy to fake it:

class Message
  include Mongoid::Document

  field :content,         type: String
  field :user_id,         type: Integer

  def user
    User.find(user_id)
  end
end

If you're not sure about the validity of your user_ids then:

def user
  User.find_by(id: user_id)
end

might work better but you'll have to be prepared for nils to come out of some_message.user. You'll probably want to add some validations on user_id to make sure it really is the id of a User.

Of course, this chicanery won't help you propagate user deletions into MongoDB or support has_many :messages in User. You'd have to do those by hand as well but an after_destroy (or around_destroy) hook and a couple simple hand-written methods (and a solid test suite) can fill in those gaps.

answered Apr 11, 2018 at 5:39

mu is too short

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2 Comments

ashvin Over a year ago

Thanks for your reply, as I have already done this, but this is not answer of my question, I want to define relationship between two model.

mu is too short Over a year ago

You can't build a cross-database relationship unless you do it by hand.

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