Mongoengine snippet related to https://stackoverflow.com/q/56609288/6203472

mongoengine_referencing_vs_embed.py

from mongoengine import connect, Document, EmbeddedDocument, \
    ReferenceField, EmbeddedDocumentListField, StringField, ObjectIdField

connect()    # Connect to `test` database by default

######
# Option1: Using 2 different collections for users and tickets
######
class User(Document):
    name = StringField()
    role = StringField()

    def get_tickets(self):
        return Ticket.objects(assignee=self)

class Ticket(Document):
    assignee = ReferenceField(User)
    content = StringField()

# Load some sample data
us1 = User(name='John', role='admin')us2 = User(name='Bob', role='guest').save()
us1.save()
Ticket(assignee=us1, content='Lorem Ipsum').save()
Ticket(assignee=us1, content='whatever').save()
Ticket(assignee=us2, content='blablabla').save()

admin_users = User.objects(role='admin')
admin_users_tickets = Ticket.objects(assignee__in=admin_users)
for ticket in admin_users_tickets:
    print(ticket.content, ticket.assignee.name, ticket.assignee.id)

# prints:
#(u'Lorem Ipsum', u'John', ObjectId('5d04dcb119dca50d61906f19'))
#(u'whatever', u'John', ObjectId('5d04dcb119dca50d61906f19'))


######
# Option2: Using 1 single collection and embedding the tickets in each user
######
class EmbedTicket(EmbeddedDocument):
    content = StringField()

class NewUser(Document):
    name = StringField()
    role = StringField()
    tickets = EmbeddedDocumentListField(EmbedTicket)

# Load sample data
NewUser(name='Malcolm', role='admin', tickets=[EmbedTicket(content='whatever1'), EmbedTicket(content='whatever2')]).save()
NewUser(name='Ulrich', role='guest', tickets=[EmbedTicket(content='whatever99')]).save()


tickets_per_admin_user = {}    # mapping {user_id: ticket}
for user in NewUser.objects(role='admin'):
    tickets_per_admin_user[str(user.id)] = user.tickets
print(tickets_per_admin_user)
# prints
# {'5d04dfee19dca50d61906f1e': [<EmbedTicket: EmbedTicket object>, <EmbedTicket: EmbedTicket object>]}

######
# Option3: same as Option2 but keeping the user_id inside the embedded tickets
# Not the one I'd recommend as it duplicates the information and isn't very handy to manipulate
######
class EmbedTicket2(EmbeddedDocument):
    content = StringField()
    user_id = ObjectIdField(required=True)

class NewUser2(Document):
    name = StringField()
    role = StringField()
    tickets = EmbeddedDocumentListField(EmbedTicket2)

us = NewUser2(name='X', role='admin').save()
us.tickets = [EmbedTicket2(content='blabla', user_id=us.id)]
us.save()

You are welcome, I'm happy to help