MongoMapper has_many association

Question

I have problem with mongomapper associations. I have one class names User and other named Model. User has many models but...

user = User.first
=> <User ...
user.models
=> []
Model.find_by_user_id(user.id.to_s)
=> <Model ...
Model.find_by_user_id(user.id.to_s).user == user
=> true

Class code (simplified):

class User
  include MongoMapper::Document

  # some keys definition

  many :models
end

class Model
  include MongoMapper::Document

  # some keys definitions

  belongs_to :user
end

What I am doing wrong?

Answer 1

It appears that MM no longer uses String format for the FK column, so

Model.find_by_user_id(user.id.to_s)

should be

Model.find_by_user_id(user.id)

Furthermore, the datatype of the Model.user_id column should be set to

key :user_id, Mongo::ObjectID

When I ran into this problem, I had to delete and recreate my collection to get it to work- in other words I used to have user_id as a String, but it would only "take" when I switched it when I rebuilt my database. Luckily I am working with test data so that was easy enough.

Answer 2

What kind of errors or exceptions are you getting? The code you posted looks fine.

Answer 3

ah, this is poorly documented in the mm docs. You need to do this here:

class User
  include MongoMapper::Document

  # some keys definition

  many :models, :in => :model_ids
end

class Model
  include MongoMapper::Document

  # some keys definitions
  # no belongs_to necessary here
end

You can then add models to your user via:

# use an existing object
u = User.create ...
m = Model.create ...

# and add the model to the user
u.models << m

# don't forget to save
u.save

# you can then check if it worked like so:
# u.model_ids  => [ BSON::ID 'your user id']

Hope that helped.