Polymorphic queries

I have replicated the example in https://ming.readthedocs.io/en/latest/polymorphism.html.

I have added a Bike and can reproduce the queries below as in the documentation.

from ming import schema
from ming.odm import FieldProperty
from ming.odm.declarative import MappedClass

class Transport(MappedClass):
    class __mongometa__:
        session = session
        name = 'transport'
        polymorphic_on = '_type'
        polymorphic_identity = 'base'

    _id = FieldProperty(schema.ObjectId)
    origin = FieldProperty(schema.String(required=True))
    destination = FieldProperty(schema.String(if_missing=''))
    _type = FieldProperty(schema.String(if_missing='base'))

    def move(self):
        return 'moving from {} to {}'.format(self.origin, self.destination)

class Bus(Transport):
    class __mongometa__:
        polymorphic_identity = 'bus'

    _type=FieldProperty(str, if_missing='bus')
    passengers_count = FieldProperty(schema.Int(if_missing=0))

    def move(self):
        return 'driving from {} to {}'.format(self.origin, self.destination)


class AirBus(Bus):
    class __mongometa__:
        polymorphic_identity = 'airbus'

    _type=FieldProperty(str, if_missing='airbus')
    wings_count = FieldProperty(schema.Int(if_missing=2))

    def move(self):
        return 'flying from {} to {}'.format(self.origin, self.destination)

class Bike(Transport):
    class __mongometa__:
        polymorphic_identity = 'bike'

    _type=FieldProperty(str, if_missing='bike')

    def move(self):
        return 'cycling from {} to {}'.format(self.origin, self.destination)


Bus(origin='Rome', destination='London', passengers_count=20))
Bus(origin='Turin', destination='London', passengers_count=20))
AirBus(origin='Turin', destination='London', passengers_count=60, wings_count=3))
Bike(origin='Newcastle', destination='London'))
session.flush()
session.clear()

print(Transport.query.find().count())  # 4
print(Transport.query.find({'_type': 'bus'}).count())  # 2
print(Transport.query.find({'_type': 'airbus'}).count()) # 1

However I have also added next queries and would expect this to produce 3 and 1:

print(Bus.query.find().count())  # expected 3, got 4
print(AirBus.query.find().count())  # expected 1, got 4

but I get 4 and 4.

Start with the polymorphism example at ming.readthedocs.io/en/latest/polymorphism.html and trace how Bus.query.find() and AirBus.query.find() build their queries. Reproduce the four inserted records and the reported counts, then identify the query behavior that makes both subclass queries return all four records. Done means the subclass counts match the documented polymorphic identities and relevant tests cover the behavior.