Using An Object Property As The $maxdistance Argument In A Mongodb Geolocation Query

I would like to write a $near query with a $maxDistance argument whose value is not constant. For example, I have a collection of fire stations with a location and a coverage radi

Solution 1:

You can't quite do this like this — in general you can not base anything in your MongoDB queries on values in the collections.

However, since MongoDB 2.4 we support a new index called 2dsphere which allows you to store not only points in the database, but polygons as well. You'd store that information in a document like:

db.so.ensureIndex( { loc: '2dsphere' } );
db.so.insert( {
    name: "Firestation 1",
    loc: {
        type: "Polygon",
        coordinates: [ [ [ 0, 0 ], [ 0, 1 ], [ 1, 1 ], [ 1, 0 ], [ 0, 0 ] ] ]
    }
} );

And then you can use an "intersect" query to figure out whether a point is covered by each of the polygons:

db.so.find( {
    'loc' : {  
        $geoIntersects: { 
            $geometry: { type: 'Point', coordinates: [ 0, 0 ] } 
        } 
    }
} );

Which then returns:

{ 
    "_id" : ObjectId("51f24d566775068ab0b786f0"), 
    "name" : "Firestation 1", 
    "loc" : { 
        "type" : "Polygon", 
        "coordinates" : [  [  [  0,  0 ],  [  0,  1 ],  [  1,  1 ],  [  1,  0 ],  [  0,  0 ] ] ] 
    } 
}

Here it finds the fire station, because 0, 0 is in the middle of the polygon. Now the trick is of course to calculate the polygon points that make up a circle that is "radius" (say 10km) away from the centre point. You won't be able to get a real circle, but a hexagon or octagon should be good enough. The maths for that isn't extremely simple, but http://www.movable-type.co.uk/scripts/latlong.html#destPoint has an example in JavaScript. Just loop your bearing in 8 steps from 0 to 2PI to calculate the points along the circumference of the circle, and put those in the coordinates. Make sure to embed them in a doubly nested array and make the first and the last one the same:

{
    name: "Firestation 1",
    loc: {
        type: "Polygon",
        coordinates: [ [
            [ point1-lon, point1-lat ], 
            [ point2-lon, point2-lat ], 
            [ point3-lon, point3-lat ], 
            ...
            [ point1-lon, point1-lat ], 
        ] ]
    }
}