Given this document:

class Article
  include Mongoid::Document

  field :details, type: Hash

  def foo=(f)
    self.details[:foo] = f
  end
end

In 2.3.4 the code snippet below works with no problem, as :details is initialized with {}:

a = Article.new
a.foo = "foo"

However in 2.4.0 it no longer works, as self.details is nil after initialization.