假设我有 3 个收藏,cars
, bikes
, vehicles
.
cars
集合为:
{
{
"_id": "car1",
"carBrand": "Audi",
"color": "blue"
},
{
"_id": "car2",
"carBrand": "BMW",
"color": "white"
}
}
bikes
集合为:
{
{
"_id": "bike1",
"bikeBrand": "Audi",
"color": "red"
},
{
"_id": "bike2",
"carBrand": "BMW",
"color": "white"
}
}
and the vehicles
集合实际上只是引用了cars
and bikes
集合为
{
{
"_id": "vehicle1",
"vehicleType": "cars",
"vehicleId": "car1"
},
{
"_id": "vehicle2",
"vehicleType": "cars",
"vehicleId": "car2"
},
{
"_id": "vehicle3",
"vehicleType": "bikes",
"vehicleId": "bike1"
},
{
"_id": "vehicle4",
"vehicleType": "bikes",
"vehicleId": "bike2"
},
}
我想加入vehicles
集合与cars
and bikes
收藏。我尝试设置"$vehicleType"
作为变量$lookup
's from
场地。然而它并没有像我预期的那样工作。它只是不加入表。没有错误。
db.collection.aggregate([{
$lookup: {
from: "$vehicleType",
localField: "vehicleId",
foreignField: "_id",
as: "vehicleDetails"
}
}]);
我期待得到这样的结果
{
{
"_id": "vehicle1",
"vehicleType": "cars",
"vehicleId": "car1",
"vehicleDetails": {
"_id": "car1",
"carBrand": "Audi",
"color": ""
}
},
{
"_id": "vehicle2",
"vehicleType": "cars",
"vehicleId": "car2",
"vehicleDetails": {
"_id": "car2",
"carBrand": "BMW",
"color": "white"
},
{
"_id": "vehicle3",
"vehicleType": "bikes",
"vehicleId": "bike1",
"vehicleDetails": {
"_id": "bike1",
"bikeBrand": "Audi",
"color": "red"
}
},
{
"_id": "vehicle4",
"vehicleType": "bikes",
"vehicleId": "bike2",
"vehicleDetails": {
"_id": "bike2",
"carBrand": "BMW",
"color": "white"
}
},
}