Define a custom field type:

class MyField
include Mongoid::Fields::Serializable

attr_accessor :my_var

def initialize(var)
@my_var = var
end

def deserialize(object)
self.class.new(object['my_var'])
end

def serialize(object)

{'my_var' => object.my_var}

end
end

and then subclass it:

class MySubfield < MyField
end

When used in a model like this:

class MyDoc
include Mongoid::Document

field :field, :type => MyField
field :subfield, :type => MySubfield
end

the subfield field is not deserialized according to the method defined in MyField:

>> x = MyDoc.new(:field => MyField.new("hello"), :subfield => MySubfield.new("world"))
>> x.field
=> #<MyField:0x1037e32c8 @my_var="hello">
>> x.subfield
=>

{"my_var" => "world"}