I want to make MongoEngine Query that just return the vehicle that i want, but it doesnt work - mongodb

Question

I'm using MongoEngine to create a query that return just vehicles that match with the constraint. As can you see Vehicle is an EmbeddedDocument and Owner is the parent document. My query is using Q, the result of the query is different that just vehicles matched with Honda brand and Element model.
from mongoengine.queryset.visitor import Q
def get_vehicles():
vehicles = json.loads(Owner.objects(Q(vehicle__brand='Honda') and Q(vehicle__model='Element')).only('vehicle').to_json())
return {"vehicles":vehicles}
class Vehicle(engine.EmbeddedDocument):
model = engine.StringField(required=True, max_length=50)
brand = engine.StringField(required=True, max_length=100)
plates = engine.StringField(required=True, max_length=50)
year = engine.IntField(required=True)
color = engine.StringField(required=True, max_length=80)
type = engine.StringField(required=True, max_length=80)
subtype = engine.StringField(required=True, max_length=80)
doors = engine.IntField(required=True)
transmission = engine.StringField(required=True, max_length=80)
photos = engine.ListField(engine.StringField(max_length=150))
equipment = engine.ListField(engine.StringField(max_length=80))
powered_by = engine.StringField(required=True, max_length=80)
availability = engine.ListField(engine.DateField())
pickup_place = engine.PointField()
creation_date = engine.DateTimeField(required=True)
class Owner(engine.Document):
_id = engine.ObjectIdField()
name = engine.StringField(required=True, max_length=100)
surname = engine.StringField(required=True, max_length=100)
gender = engine.StringField(required=True, max_length=1)
birthday = engine.DateField(required=True)
creation_date = engine.DateTimeField(required=True)
last_update = engine.DateTimeField()
photo = engine.StringField(required=True, max_length=200)
access = engine.EmbeddedDocumentField(Access)
address = engine.EmbeddedDocumentField(Address)
vehicle = engine.EmbeddedDocumentListField(Vehicle)

Answer 1

i'm near to the result that i want, but i need to add the result the owner id to vehicle.
def get_vehicles():
matched_owners = json.loads(Owner.objects(Q(vehicle__brand='VW') and Q(vehicle__model='Jetta')).only('vehicle').to_json())
vehicles = [ vehicle for owner in matched_owners for vehicle in owner['vehicle'] if vehicle['brand'] == "VW" and vehicle['model'] == "Jetta"]
return {"vehicles":vehicles}
The query on Mongo shell that match with the result that i want is:
db.owner.find({"vehicle.brand":"VW"},{vehicle:{$elemMatch:{model:"Jetta"}}});

Answer 2

I get the result that i want on MongoShell
db.owner.aggregate([{ $unwind:"$vehicle"}, {$match:{ "vehicle.model":"Jetta", "vehicle.brand":"VW"}}, {$project: {"vehicle.model":1, "vehicle.brand":1, "vehicle.color":1, "vehicle.doors":1, "vehicle.transmission":1, "vehicle.powered_by":1 ,"vehicle.photos":1, "vehicle.equipment":1, "vehicle.availability":1, "vehicle.pickup_place":1}} ]);```
i attempted to do the same on MongoEngine, but the result is different:
```python
def get_vehicles():
pipeline = [
{ "$unwind":"$vehicle"},
{
"$match":{ "vehicle.model":"Jetta", "vehicle.brand":"VW"}
},
{
"$project": {"vehicle.model":1, "vehicle.brand":1, "vehicle.color":1, "vehicle.doors":1, "vehicle.transmission":1, "vehicle.powered_by":1 ,"vehicle.photos":1, "vehicle.equipment":1, "vehicle.availability":1, "vehicle.pickup_place":1}
}
]
vehicles = Owner.objects.aggregate(pipeline)
return {"vehicles":vehicles}```

Answer 3

Indeed you are querying the parent collection (Owner) so the query returns all owner document with any of the Vehicle matching your Q conditions.
Although not elegant, adding a second filtering pass in Python will do the trick:
from mongoengine import *
connect()
class Vehicle(EmbeddedDocument):
model = StringField(required=True, max_length=50)
brand = StringField(required=True, max_length=100)
def __repr__(self):
return f'Vehicle({self.brand} - {self.model})'
class Owner(Document):
vehicles = EmbeddedDocumentListField(Vehicle)
Owner(vehicles=[Vehicle(model="Civic", brand="Honda"), Vehicle(model="Element", brand="Honda")]).save()
Owner(vehicles=[Vehicle(model="Golf", brand="VW")]).save()
def get_vehicles(brand, model):
matched_owners = Owner.objects(Q(vehicles__brand=brand) and Q(vehicles__model=model)).only('vehicles')
vehicles = [vehicle for owner in matched_owners for vehicle in owner.vehicles if vehicle.brand == brand and vehicle.model == model]
return vehicles
get_vehicles("Honda", "Element") # Print [Vehicle(Honda - Element)]
What you want is probably feasible using an aggregation pipeline but if your use case is really this one, you'll be just fine with this
Off topic but I'd recommend you to avoid defining a '_id' attribute, MongoEngine gives you a id = ObjectIdField() field by default and uses "_id" attribute internally so it's not a good idea to overwrite it (main MongoEngine contributor here)