How to reference existing documents in collection on creation of documents in another collection MongoEngine

I'm using a model as follows:

# Creating data models for the interns first project.
class Sex(db.Document):
    _id = ObjectIdField(required=True, default=ObjectId, unique=True, primary_key=True)
    gender = db.StringField()


class Address(db.EmbeddedDocument):
    _id = ObjectIdField(required=True, default=ObjectId, unique=True, primary_key=True)
    number = db.IntField()
    street = db.StringField()
    city = db.StringField()
    eircode = db.StringField()


class Person(db.Document):
    _id = ObjectIdField(required=True, default=ObjectId, unique=True, primary_key=True)
    name = db.StringField()
    sex = db.ReferenceField(Sex, required=True)
    address = db.ListField(db.EmbeddedDocumentField(Address))

I already have the values created as documents in 'sex' collection, I want to reference to one of those values on the insertion of a new document in 'person' collection. (As far as I understood that would be a one to many relationship)

That's how far I got but that doesn't work as expected:

    if Person(name='USERNAME', sex=Sex(_id='602a593d3149eb362c296561', gender='Male'),
              address=[Address(number=1, street='STREET 1', city='CITY 1', eircode='EIRCODE 1'),
                       Address(number=2, street='STREET 2', city='CITY 2', eircode='EIRCODE 2')]
              ).save():
        return 'Successfully added'
    return 'Error in Register'

Found out that I was getting an error for setting default values on the oid (_id) field in person, worked fine after removing it.

# Creating data models for the interns first project.
class Sex(db.Document):
    _id = ObjectIdField(required=True, default=ObjectId, unique=True, primary_key=True)
    gender = db.StringField()


class Address(db.EmbeddedDocument):
    _id = ObjectIdField(required=True, default=ObjectId, unique=True, primary_key=True)
    number = db.IntField()
    street = db.StringField()
    city = db.StringField()
    eircode = db.StringField()


class Person(db.Document):
    _id = ObjectIdField()
    name = db.StringField()
    sex = db.ReferenceField(Sex, required=True)
    address = db.ListField(db.EmbeddedDocumentField(Address))