class Image
  include Mongoid::Document
  embedded_in :page

  def bam= value
    puts 'calling bam='
    byebug
    puts "page: #{page}" # This is nil. Incorrect.
  end
end

class Page
  include Mongoid::Document
  embeds_many :images, cascade_callbacks: true
  accepts_nested_attributes_for :images
end

attributes = {
  'images_attributes' => {
    '0' => {
      'bam' => 'bam'
    }
  }
}
page = Page.new
page.attributes = attributes

puts 'after setting attributes'
puts page.images.first.page