This module abstracts the functionality for generating sequential unique integer IDs for instances of the class. It defines the method next_id on the class that includes it. The implementation ensures that the IDs will be unique even when called from multiple threads.

@example Include the Id module.

  class Foo
    include Mongo::Id
  end

  f = Foo.new
  foo.next_id # => 1
  foo.next_id # => 2

Classes which include Id should not access `@@id` or `@@id_lock` directly; instead, they should call `next_id` in `initialize` and save the result in the instance being created.

@example Save the ID in the instance of the including class.

  class Bar
    include Mongo::Id

    attr_reader :id

    def initialize
      @id = self.class.next_id
    end
  end

  a = Bar.new
  a.id # => 1
  b = Bar.new
  b.id # => 2

@since 2.7.0 @api private