lookup with pipeline and geoIntersects that use let variable - mongodb

Question

I am trying to lookup for neighborhoods which match my condition - the boundries polygon intersects with the post's coordinates but I am unable to do it - cant use the let in my pipeline $match
example of post entity:
{
_id: ObjectId,
...,
location: {
...,
coordinates: {
type: 'Point',
coordinates: [number, number]
}
}
};
example of neighborhood entity:
{
_id: ObjectId,
...,
boundries: {
type: 'Polygon',
coordinates: [ [ [number, number], [number, number], [number, number], [number, number], [number, number] ] ]
}
};
example of query I am trying to "fix":
db.posts.aggregate([
{ $match: { _id: ObjectId('5a562e62100338001218dffa') } },
{
$lookup: {
from: 'neighborhoods',
let: { postCoordinates: '$location.coordinates.coordinates' },
pipeline: [
{
$match: {
boundries: {
$geoIntersects: {
$geometry: {
type: 'Point',
coordinates: '$$postCoordinates'
}
}
}
}
}
],
as: 'neighborhoods'
}
}
]);

Answer 1

Unfortunately coordinates can't be populated from document field.
Geospatial are query expressions and $let variables are only permitted to use in $match with $expr variant for aggregation expressions in $lookup pipeline.
You have to perform the query in two steps.
First step to get the coordinates for matching record.
var result = db.posts.findOne({ _id: ObjectId('5a562e62100338001218dffa')},{ _id: 0, 'location':1});
Second step to look for point in the polygon.
db.neighborhoods.find(
{
"boundries": {
$geoIntersects: {
$geometry: {
type: "Point" ,
coordinates: result.location.coordinates.coordinates
}
}
}
}
)