How to use $geoNear after $lookup - mongodb

Question

Hi i have the following problem:
I would like to use $geoNear (to count distance between two points) but after $loopback (and on collection that i joined).
This is the model for companyBases collection (i would like to join it):
{
"_id" : ObjectId("5d7cfe13f42e7345d967b378"),
"location" : {
"type" : "Point",
"coordinates" : [
20.633856,
49.761268
]
},
"vehicles" : [
{
"_id" : ObjectId("5d7cfe13f42e7345d967b340"),
...other fields that doesn't matter
}
]
}
This is vehicle collection:
{
"_id" : ObjectId("5d7cfe13f42e7345d967b340"),
...other fields that doesn't matter
}
I would like to join companyBase collection in aggregation on vehicles collection:
db.vehicles.aggregate([
{
$lookup: {
from: "companybases",
let: {
vehicleId: "$_id"
},
pipeline: [
{
$match: {
$expr: { $in: ["$$vehicleId", "$vehicles._id"] }
}
}
],
as: "companyBases"
}
},
{
$unwind: "$companyBases"
},
{
$geoNear: {
near: {
type: "Point",
coordinates: [50.02485, 20.0008]
},
distanceField: "distance",
spherical: true
}
}
]);
But it returns me:
{
"message" : "$geoNear is only valid as the first stage in a pipeline.",
"operationTime" : "Timestamp(1568472833, 1)",
"ok" : 0,
"code" : 40602,
"codeName" : "Location40602",
"$clusterTime" : {
"clusterTime" : "Timestamp(1568472833, 1)",
"signature" : {
"hash" : "AAAAAAAAAAAAAAAAAAAAAAAAAAA=",
"keyId" : "0"
}
},
"name" : "MongoError"
}
When i am doing the same pipeline on companybases collection it returns me documents with counted distance:
db.companybases.aggregate([
{
$geoNear: {
near: {
type: "Point",
coordinates: [50.02485, 20.0008]
},
distanceField: "distance",
spherical: true
}
}
]);
And result:
{
"_id" : ObjectId("5d7cfe13f42e7345d967b378"),
"location" : {
"type" : "Point",
"coordinates" : [
20.633856,
49.761268
]
},
"vehicles" : [
{
...some fields
},
],
...some fields
"distance" : 4209673.447019393
}
I realize that the error may be because of missing index on vehicles collection. So is there any way to calculate distance with $geoNear with $lookup ? Or maybe it's impossible and i have to do on my own ?

Answer 1

Simple solutions (you can put $geoNear in $lookup pipeline):
db.vehicles.aggregate([
{
$lookup: {
from: "companybases",
let: {
vehicleId: "$_id"
},
pipeline: [
{
$geoNear: {
near: {
type: "Point",
coordinates: [50.02485, 20.0008]
},
distanceField: "distance",
spherical: true
}
},
{
$match: {
$expr: { $in: ["$$vehicleId", "$vehicles._id"] }
}
}
],
as: "companyBases"
}
},
{
$unwind: "$companyBases"
}
]);
But that strongly impressed the performance (it takes at least 5 seconds), becuase $geoNear is used before match.

Answer 2

your error shows "message" : "$geoNear is only valid as the first stage in a pipeline.",
it is clear that you can't put '$geoNear' after $lookup
so You can only use $geoNear as the first stage of a pipeline.
just to exchange you agg order
use $geoNear at first then with $lookup