require 'mongoid'

Mongoid.configure do |c|
  c.clients.default = { hosts: ['localhost:27017'], database: 'mongoid_embed_repro' }
end

puts "Mongoid version:  #{Mongoid::VERSION}"

class Parent
  include Mongoid::Document
  include Mongoid::Timestamps
  embeds_one :child
end

class Child
  include Mongoid::Document
  field :label, type: String
  embedded_in :parent
end

Parent.delete_all

parent = Parent.create!                       # persisted, has updated_at + created_at
child  = parent.create_child(label: "c1")     # bug: embedded_in defaults to touch: true

puts ""
puts "ruby-side:    child.persisted? = #{child.persisted?}"
puts "ruby-side:    parent.child     = #{parent.child.inspect[0,80]}"

raw = Parent.collection.find(_id: parent.id).first
puts "db-side:      raw keys         = #{raw.keys.inspect}"
puts "db-side:      raw['child']     = #{raw['child'].inspect}"

parent.reload
puts "post-reload:  parent.child     = #{parent.child.inspect[0,80]}"

# Expected:
#   raw['child']             => the embedded doc as BSON
#   parent.reload.child      => Child instance
# Actual on mongoid master 9.1.0:
#   raw['child']             => nil
#   parent.reload.child      => nil
#   child.persisted?         => true (still - silent failure)

Mongoid version: 9.1.0
ruby-side: child.persisted? = true
ruby-side: parent.child = #<Child _id: 6a02a19c96570055c2a06135, label: "c1">
db-side: raw keys = ["_id", "updated_at", "created_at"]
db-side: raw['child'] = nil
post-reload: parent.child = nil