A rough solution to https://jira.mongodb.org/browse/MONGOID-3042

mongoid_custom_array.rb

module MongoidEmbeddedObjectModification

  # Custom field behaves more like an immutable value.
  # Your changes to the object obtained with getter method will not take an effect unless you assign it back.
  # https://github.com/mongoid/mongoid/issues/3341
  def modify(mongoid_doc, field_name)
    obj = mongoid_doc.public_send(field_name)
    yield obj
    mongoid_doc.public_send("#{field_name}=", obj)
  end

end

# ================

class MongoidCustomArray < DelegateClass(Array)
  extend MongoidEmbeddedObjectModification

  module ArrayClasses

    class << self

      def find_or_add_for(type)
        classes[type] ||= add_class(yield)
      end

      private

      def classes
        @classes ||= {}
      end

      def add_class(new_klass)
        type_name = new_klass::ELEMENT_TYPE.name.demodulize
        name = "ArrayOf#{type_name}"
        if const_defined?(name)
          i = 1;
          begin
            i += 1
            name = "Array#{i}Of#{type_name}"
          end while const_defined?(name)
        end
        const_set(name, new_klass)
        new_klass
      end

    end

  end

  def self.of(type)
    ArrayClasses.find_or_add_for(type) do
      Class.new(self) do
        const_set(:ELEMENT_TYPE, type)
      end
    end
  end

  module Mongoization
    extend ActiveSupport::Concern

    module ClassMethods

      def demongoize(object)
        case object
        when Array
          self[*object.map(&self::ELEMENT_TYPE.public_method(:demongoize))]
        when nil
          nil
        end
      end

      def mongoize(object)
        object.map(&self::ELEMENT_TYPE.public_method(:mongoize))
      end

      def evolve(object)
        mongoize(object)
      end

    end

    def mongoize
      self.class.mongoize(self)
    end

  end
  include Mongoization

  def self.[](*elms)
    self.new(elms)
  end

end

What you do:

Note Point class uses MongoidEmbeddedObject from https://gist.github.com/ippeiukai/df75196427c869454886

class Point
  include MongoidEmbeddedObject
  def self.at(x,y)
    self.new(x: x, y: y)
  end
  field :x, type: Integer, default: 0
  field :y, type: Integer, default: 0
end

class Profile
  include Mongoid::Document
  field :locations, type: MongoidCustomArray.of(Point)
end

What you get:

profile = Profile.new(locations:[Point.at(0,0), Point.at(12,24)])
MongoidCustomArray.modify(profile, :locations) do |locations|
  locations.first.x = 10
end
puts profile.inspect # => #<Profile _id: 545c61a34e3132c711000000, locations: [{"x"=>10, "y"=>0}, {"x"=>12, "y"=>24}]>
profile.save

profile = Profile.last
profile.locations # => [#<Point x: 10, y: 0>, #<Point x: 12, y: 24>]