Question

I have three simple models – Car, Details and Details2.
Car have lot of Details and Details have lot of Details2.

With MySQL and ActiveRecord I will set the associations like:

class Car
  has_many :details
end


class Detail
   belongs_to :car
end

In the view, where I have a list of Car, I have:

<% @cars.each do |car| -%>
  <%=car.details.count%> #uninitialized constant Details
<% end %>

EDIT2:
This is what I have in cars table:

db.cars.find()
{ "_id" : ObjectId("4efe69716f85ce447a000054"), "name" : "bmw", "descr" : "asasgasga as gas gas ", "updated_at" : "Sat Dec 31 2011 02:46:25 GMT+0100 (CET)", "created_at" : "Sat Dec 31 2011 02:46:25 GMT+0100 (CET)" }

and details:

db.details.find()
{ "_id" : ObjectId("4f01106d6f85ce6b850000b8"), "car_id" : ObjectId("4efe69716f85ce447a000054"), "name" : "20120102030325", "descr" : "dsg", "updated_at" : "Mon Jan 02 2012 03:03:25 GMT+0100 (CET)", "created_at" : "Mon Jan 02 2012 03:03:25 GMT+0100 (CET)" }

And in CarsController I have in this action only:

def index
  @cars = Car.all
end

that’s all what I do.

Answer 1

You can reuse what you know from ActiveRecord. has_many and belongs_to work just fine in Mongoid.

But you can take advantage of document databases (which MongoDB is) and use some embedding. For embedded_in to work correctly you have to have embeds_one or embeds_many on the other end. See http://mongoid.org/docs/relations/embedded/1-1.html

  class Person
    include Mongoid::Document
    embeds_one :name
  end

  class Name
    include Mongoid::Document
    field :vorname, type: String
    field :nachname, type: String
    embedded_in :person
  end