Mongoid: unique index for embedded documents - ruby

Question

I'm trying to create a unique field for embedded documents:
class Chapter
include Mongoid::Document
field :title
end
class Book
include Mongoid::Document
field :name
embeds_many :chapters
index({ 'name' => 1 }, { unique: true })
index({ 'name' => 1, 'chapters.title' => 1 }, { unique: true, sparse: true })
# index({ 'name' => 1, 'chapters.title' => 1 }, { unique: true })
end
I run the task: rake db:mongoid:create_indexes
I, [2017-02-22T08:56:47.087414 #94935] INFO -- : MONGOID: Created indexes on Book:
I, [2017-02-22T08:56:47.087582 #94935] INFO -- : MONGOID: Index: {:name=>1}, Options: {:unique=>true}
I, [2017-02-22T08:56:47.087633 #94935] INFO -- : MONGOID: Index: {:name=>1, :"chapters.title"=>1}, Options: {:unique=>true, :sparse=>true}
But it doesn't work as I would expect...
Book.new( name: 'A book', chapters: [ { title: 'title1' }, { title: 'title1' }, { title: 'title2' } ] ).save # no errors
Book.new( name: 'Another book', chapters: [ { title: 'title2' } ] ).save
b = Book.last
b.chapters.push( Chapter.new( { title: 'title2' } ) )
b.save # no errors
Any idea?
UPDATE: Ruby 2.4.0, Mongo 3.2.10, Mongoid 5.2.0 | 6.0.3 (trying both)
UPDATE2: I add also the tests I made directly with mongo client:
use books
db.books.ensureIndex({ title: 1 }, { unique: true })
db.books.ensureIndex({ "title": 1, "chapters.title": 1 }, { unique: true, sparse: true, drop_dups: true })
db.books.insert({ title: "Book1", chapters: [ { title: "Ch1" }, { title: "Ch1" } ] }) # allowed?!
db.books.insert({ title: "Book1", chapters: [ { title: "Ch1" } ] })
b = db.books.findOne( { title: 'Book1' } )
b.chapters.push( { "title": "Ch1" } )
db.books.save( b ) # allowed?!
db.books.findOne( { title: 'Book1' } )
db.books.insert({ title: "Book2", chapters: [ { title: "Ch1" } ] })
UPDATE3: I made more tests but I didn't succeed, this link helped but the problem remains

Answer 1

You should use drop_dups
class Category
include Mongoid::Document
field :title, type: String
embeds_many :posts
index({"posts.title" => 1}, {unique: true, drop_dups: true, name: 'unique_drop_dulp_idx'})
end
class Post
include Mongoid::Document
field :title, type: String
end
Rails console:
irb(main):032:0> Category.first.posts.create(title: 'Honda S2000')
=> #<Post _id: 58adb923cacaa6f778215a26, title: "Honda S2000">
irb(main):033:0> Category.first.posts.create(title: 'Honda S2000')
Mongo::Error::OperationFailure: E11000 duplicate key error collection: mo_development.posts index: title_1 dup key: { : "Honda S2000" } (11000)