I've had this issue a few times now, where I'm calling .only() for efficiency, and then when using the value, I don't get the default, I get None returned.

Other fields not specified in the .only() function do get set to their default value, however.

Reproduce:

class Example(Document):
    meta = {"collection": "example"}

    str1 = StringField(default="hi")
    str2 = StringField(default="hi2")

Example().save()
a = Example.objects.only("str1").first() # a.str1 == None, a.str2 == "hi2"