MongoDB and geospatial queries: unknown geo specifier: $geometry: null

Question

i'm experimenting with mongodb and geospatial queries. I made the following collection:

shape> db.shape.find()
[
  {
    _id: ObjectId("61ab50d2056b5357b5e23e56"),
    name: 'Point1',
    structure: { type: 'Point', coordinates: [ 2.5, 2.5 ] }
  },
  {
    _id: ObjectId("61ab5337056b5357b5e23e57"),
    name: 'Point2',
    structure: { type: 'Point', coordinates: [ 5, 5 ] }
  },
  {
    _id: ObjectId("61ab533e056b5357b5e23e58"),
    name: 'Point3',
    structure: { type: 'Point', coordinates: [ 9, 9 ] }
  },
  {
    _id: ObjectId("61ab5b4d056b5357b5e23e64"),
    name: 'square',
    structure: {
      type: 'Polygon',
      coordinates: [ [ [ 0, 0 ], [ 0, 5 ], [ 5, 5 ], [ 5, 0 ], [ 0, 0 ] ] ]
    }
  }
]

then i tried the following:

var square = db.shape.find({name :"square"});
db.shape.find({structure : { $geoWithin : { $geometry : square.structure}}});

and i get this error:

MongoServerError: unknown geo specifier: $geometry: null

why is $geometry null?

Thanks in advance

Answer 1

While processing the query, square.structure is getting replaced with null because you are trying to access that variable in wrong way.

In that square variable you are storing results returned by find() method.

find() method returns a cursor and we need to use cursor methods (like forEach) to access the containing documents. Where as findOne() returns a single document which can be accessed directly.

So, you can:

1. Use findOne() instead of find() or
2. Use cursor method like square.forEach(function(mydoc){<iterate through cursor here>})

I have copied your data and tested your query with findOne() and it worked well.