Because a default scope initializes fields in new models as just described,
defining a default scope with a dotted key and a simple literal value is
not possible:

.. code-block:: ruby

  class Band
    include Mongoid::Document
    field :name, type: String
    field :tags, type: Hash

    default_scope ->{ where('tags.foo' => 'bar') }
  end

  Band.create! # exception: BSON::String::IllegalKey ('tags.foo' is an illegal key in MongoDB. Keys may not start with '$' or contain a '.'.)