mongoid callbacks with relation

ab.rb

class A
  include Mongoid::Document

  field :hoge, type: String

  belongs_to :b

  before_validation do
    p 'before_validation'
  end
  before_validation on: :create do
    p 'before_validation on create'
  end
  before_validation on: :update do
    p 'before_validation on update'
  end
  after_validation on: :create do
    p 'after_validation on create'
  end
  after_validation on: :update do
    p 'after_validation on update'
  end
  before_create do
    p 'before_create'
  end
  before_update do
    p 'before_update'
  end
  before_save on: :create do
    p 'before_save on create'
  end
  before_save on: :update do
    p 'before_save on update'
  end
end


class B
  include Mongoid::Document

  field :fuga, type: String

  has_one :a
end

__END__

# mongoid (6.1.1)

> b = B.create(fuga: "b")
> a = A.create(hoge: "a", b: b)
"before_validation"
"before_validation on create"
"after_validation on create"
"before_save on create"
"before_save on update"
"before_create"
> b.fuga = "bb"
> b.save
"before_validation"
"before_validation on update"
"after_validation on update"

It is important to note that by default, Mongoid will validate the children of any relation that are loaded into memory via a validates_associated. The relations that this applies to are:
embeds_many
embeds_one
has_many
has_one
has_and_belongs_to_many
https://mongoid.github.io/old/en/mongoid/docs/relations.html