Intersept a FeatureCollection in MongoDB - json

Question

I have a GeoJson filled with states from Austria and I want to do a query that gives me as output which certain states intercepts my polygon.
This is my query:
db.GeoAustria.find(
{
'features.geometry':{
$geoIntersects:{
$geometry:{
type: "Polygon",
coordinates: [
[
[
16.21685028076172,
48.007381433478855
],
[
16.24225616455078,
47.98716432210271
],
[
16.256675720214844,
48.00669234420252
],
[
16.21685028076172,
48.007381433478855
]
]
]
}
}
}
}
)
But it gives me all the features, including those that don't overlap the polygon...
Where is my mistake in this query?

Answer 1

Basic array match misunderstanding here. The input set is a single doc with 95 polygons in an array in a single FeatureCollection object. When you do a find() on such things, any individual geo that is an intersect will cause the entire doc to be returned as a match. This is exactly the same as:
> db.foo.insert({x:["A","B","C"]})
WriteResult({ "nInserted" : 1 })
> db.foo.find({x:"A"});
{ "_id" : ObjectId("5fb1845b08c09fb8dfe8d1c1"), "x" : [ "A", "B", "C" ] }
The whole doc is returned, not just element "A".
Let's assume that you might have more than one big doc in your collection. This pipeline yields the single target geometry for Baden (I tested it on your input set):
var Xcoords = [
[
[
16.21685028076172,
48.007381433478855
],
[
16.24225616455078,
47.98716432210271
],
[
16.256675720214844,
48.00669234420252
],
[
16.21685028076172,
48.007381433478855
]
]
];
var targ = {type: "Polygon", coordinates: Xcoords};
db.geo1.aggregate([
// First, eliminate any docs where the geometry array has zero intersects. In this
// context, features.geometry means "for each element of array features get the
// geometry field from the object there", almost like saying "features.?.geometry"
{$match: {"features.geometry": {$geoIntersects: {$geometry: targ}} }}
// Next, break up any passing docs of 95 geoms into 95 docs of 1 geom...
,{$unwind: "$features"}
// .. and run THE SAME $match as before to match just the one we are looking for.
// In this context, the array is gone and "features.geometry" means get JUST the
// object named geometry:
,{$match: {"features.geometry": {$geoIntersects: {$geometry: targ}} }}
]);
Beyond this, I might recommend breaking up that FeatureCollection into something that is both indexable (FeatureCollection is NOT indexable in MongoDB) and easier to deal with. For example, this little script run against your single-doc/many-polys design will convert it in 95 docs with extra info:
db.geo2.drop();
mainDoc = db.geo1.findOne(); // the one Austria doc
mainDoc['features'].forEach(function(oneFeature) {
var qq = {
country: "Austria",
crs: mainDoc['crs'],
properties: oneFeature['properties'],
geometry: oneFeature['geometry']
};
db.geo2.insert(qq);
});
db.geo2.aggregate([
{$match: {"geometry": {$geoIntersects: {$geometry: targ}} }}
]);
// yields same single doc output (Baden)
This allows ease of matching and filtering. For more on FeatureCollection vs. GeometryCollection see https://www.moschetti.org/rants/hurricane.html.