How to save embeds_many relation in Mongoid?

Question

class Hotel
  include Mongoid::Document

  field :title, type: String

  embeds_many :comments
end

class Comment
  include Mongoid::Document

  field :text, type: String

  belongs_to :hotel

  validates :text, presence: true
end

h = Hotel.create('hotel') 

   => <#Hotel _id: 52d68dd47361731d8b000000, title: "hotel">

c = Comment.new(text: 'text')

   => <#Comment _id: 52d68f3d7361731d8b040000, text: "text", hotel_id: nil>

h.comments << c

   => [#<Comment _id: 52d68f3d7361731d8b040000, text: "text", hotel_id: nil>]
h.save

   => true
Hotel.last.comments

   => []

variant 2

h.comments << Comment.new(text: 'new', hotel_id: h.id)

   => [<#Comment _id: 52d68f3d7361731d8b040000, text: "text", hotel_id: nil>, <#Comment _id: 52d691e17361731d8b050000, text: "new", hotel_id: BSON::ObjectId('52d68dd47361731d8b000000')>]

h.save

   => true
Hotel.last.comments

   => []

Answer 1

I see two possible problems:

1. Hotel.last is not necessarily Hotel 52d68dd47361731d8b000000. You should look at h.comments or to be paranoid, h.reload and h.comments.
2. Your associations are confused.

From the fine manual:

Embedded 1-n

One to many relationships where the children are embedded in the parent document are defined using Mongoid's embeds_many and embedded_in macros.

Defining

The parent document of the relation should use the embeds_many macro to indicate it has n number of embedded children, where the document that is embedded uses embedded_in.

So your relation should be defined like this:

class Hotel
  embeds_many :comments
end

class Comment
  embedded_in :hotel
end

You're using belongs_to: hotel in Comment when you should say embedded_in :hotel.

The docs also say that:

Definitions are required on both sides to the relation in order for it to work properly.

and your relation is incorrectly configured on one side so it won't work properly.