Starting with Mongoid 8, creating an associated document via build doesn't call its initialize method anymore.

Here's a small example: (setting a default value is just an example to illustrate the actual problem of initialize being skipped)

class Foo
  include Mongoid::Document
  embeds_many :bars
end

class Bar
  include Mongoid::Document
  embedded_in :foo
  field :name, type: String

  def initialize(...)
    super
    self.name ||= "default"
  end
end

The above code results in:

foo = Foo.new

foo.bars << Bar.new
foo.bars
#=> [#<Bar _id: 64392e9c571de67659233fe7, name: "default">]

foo.bars.build
foo.bars
#=> [#<Bar _id: 64392e9c571de67659233fe7, name: "default">,
#    #<Bar _id: 64392e9c571de67659233fe8, name: nil>]

Note that the Bar instance created via build lacks a value for name. Prior to Mongoid 8, both variants behaved identically.

 initialize is a crucial method in Ruby's object lifecycle and shouldn't be skipped.