from mongoengine import connect, Document, StringField, ListField, ReferenceField
from pathos.multiprocessing import ProcessingPool


class Base(Document):
    key = StringField(primary_key=True)
    name = StringField()
    parent = ReferenceField('Parent', required=True)

class Parent(Document):
    key = StringField(primary_key=True)
    name = StringField()
    bases = ListField(ReferenceField('Base'))


def remove_base(key):
   db = connect('mydb')
   mongo_b = Base.objects().get(key=key)
   mongo_b.parent.update(pull__bases=mongo_b)
   mongo_b.delete()


### setup
db = connect('mydb', connect=False)

Base(key='b1', name='test', parent='p1').save()
Base(key='b2', name='test', parent='p1').save()
Base(key='b3', name='test2', parent='p1').save()

p=Parent(key='p1', name='parent').save()
p.update(add_to_set__bases='b1')
p.update(add_to_set__bases='b2')
p.update(add_to_set__bases='b3')

### find objects we want to delete
my_base_objects = Base.objects(name='test')
keys = [b.key for b in my_base_objects]
del my_base_objects

# close db to avoid problems?!
db.close()
db = None

# parallel map removing base objects and references from the db
# warning generated here
pp = ProcessingPool(2)
pp.map(remove_base, keys)