mongodb - mongoengine.EmbeddedDocument クラスで save メソッドを呼び出す方法

Question

import mongoengine

class Model1(mongoengine.DynamicDocument):
    name = mongoengine.StringField()
    addr = mongoengine.EmbeddedDocumentField(Model2)

class Model2(mongoengine.EmbeddedDocument):
    loc =  mongoengine.StringField()
    # do some stuff
    def save(self, *args, **kwargs):
        print "test line print...."
        super(Model2, self).save(*args, **kwargs)

Model1インスタンスを保存するとき。save メソッドを呼び出さない

m2 = Model2(loc='some text')
m1 = Model1(name='name')
m1.addr = m2
m1.save()

Model2 で save メソッドを明示的に呼び出そうとすると、NoneType object has no attr save