MongoDB: Matching points from one collection with polygons from another - mongodb

Question

I'm trying to match points in one collection with regions stored in another collection.
Here are examples of documents.
Points:
{
"_id" : ObjectId("5e36d904618c0ea59f1eb04f"),
"gps" : { "lat" : 50.073288, "lon" : 14.43979 },
"timeAdded" : ISODate("2020-02-02T15:13:22.096Z")
}
Regions:
{
"_id" : ObjectId("5e49a469afae4a11c4ff3cf7"),
"type" : "Feature",
"geometry" : {
"type" : "Polygon",
"coordinates" : [
[
[ -748397.88, -1049211.61 ],
[ -748402.77, -1049212.2 ],
...
[ -748410.41, -1049213.11 ],
[ -748403.05, -1049070.62 ]
]
]
},
"properties" : {
"Name" : "Region 1"
}
}
And the query I'm trying to construct is something like this:
db.points.aggregate([
{$project: {
coordinates: ["$gps.lon", "$gps.lat"]
}},
{$lookup: {
from: "regions", pipeline: [
{$match: {
coordinates: {
$geoWithin: {
$geometry: {
type: "Polygon",
coordinates: "$geometry.coordinates"
}
}
}
}}
],
as: "district"
}}
])
I'm getting an error:
assert: command failed: {
"ok" : 0,
"errmsg" : "Polygon coordinates must be an array",
"code" : 2,
"codeName" : "BadValue"
} : aggregate failed
I've noticed the structure of $geoWithin document is same as structure of one I have for each region. So I tried such query:
db.points.aggregate([
{$project: {
coordinates: ["$gps.lon", "$gps.lat"]
}},
{$lookup: {
from: "regions", pipeline: [
{$match: {
coordinates: {
$geoWithin: "$geometry.coordinates"
}
}}
],
as: "district"
}}
])
The error was same.
I looked up for geoqueries but surprisingly all found mentions had static region document instead of one taken from a collection. So I'm wondering - is it ever possible to map points with regions having that both document collections aren't static and taken from DB?

Answer 1

Unfortunately not possible
You could perform query below if $geometry could deal with MongoDB Aggregation Expressions.
db.points.aggregate([
{
$lookup: {
from: "regions",
let: {
coordinates: [
"$gps.lon",
"$gps.lat"
]
},
pipeline: [
{
$addFields: {
coordinates: "$$coordinates"
}
},
{
$match: {
coordinates: {
$geoWithin: {
$geometry: {
type: "Polygon",
coordinates: "$geometry.coordinates"
}
}
}
}
}
],
as: "district"
}
}
])