Mongoid::Errors::MixedRelations

问题

I've a User model embedding "one to many" Watchlists like the following :

class User 
  include Mongoid::Document

  field :uid
  field :name     
  field :user_hash

  embeds_many :watchlists
end


class Watchlist
  include Mongoid::Document

  field :html_url
  field :description

  #field :name
  field :fork_, :type => Boolean

  field :forks, :type => Integer
  field :watchers, :type => Integer

  field :created_at, :type => DateTime
  field :pushed_at, :type => DateTime

  field :avatar_url

  embedded_in :user
  has_and_belongs_to_many :tags
end

The Watchlist should also references a many to many Tag model and vice versa :

class Tag
  include Mongoid::Document
  field :name, type: String

  has_and_belongs_to_many :watchlists
end

Anyway, that's causing an error and seems that kind of "mixed" relation is not possible :

Mongoid::Errors::MixedRelations (Referencing a(n) Watchlist document from the Tag document via a relational association is not allowed since the Watchlist is embedded.):
 app/controllers/home_controller.rb:53:in `tagging'

UPDATE Please note that watchlist, has to be dropped (user.watchlists.clear) than re-created (user.watchlists.find_or_create_by) four times a day, while Tag/s have to be persistent, relating the same embedded watchlists as before ( ... I'm not sure that is possible anyway, because of previous drop/creation ).

UPDATE of UPDATE ( tanks to durran support ) No, that's not possible: If you clear the embedded docs, the ids are gone as well, and new ones will get generate each time you create a new one.

Do you have any idea on how to overcome that ? Is it better to split all three models in referenced relations ( three different collections )?

回答1:

In mongoid you can't have references to embedded documents. So the problem is in your tag model defining habtm there. You can have HABTM in embedded watchlists, without any inverse relation.

class User 
  include Mongoid::Document
  embeds_many :watchlists
end

class Watchlist
  include Mongoid::Document
  embedded_in :user
  has_and_belongs_to_many :tags, inverse_of: nil
end

class Tag
  include Mongoid::Document
end

But if you must have references to watchlists in tags, you can manually maintain array of ids on both sides as already pointed out by Tyler.

回答2:

Not the answer you are looking for but... for what it's worth, whenever I used HABTM relations in mongoid it either was buggy or didn't work. I don't know if this has been fixed but if you stick to just using arrays in the models and also on the reverse side of the relation you should be golden. Thats pretty much what the code would do for you anyway.

Yes, you have to do a bit more work to maintain the relationships but it actually works like it's supposed to.

Good luck.

来源：https://stackoverflow.com/questions/8437181/mongoiderrorsmixedrelations

标签

ruby-on-rails

mongodb

mongoid