Golang-mgo中的$ literal用法

Question

I can't understand how to properly use $literal. I am using mgo.v2 and mgo.v2/bson package.

db.store.aggregate([
{"$project":{
    "location":{
        "type":{"$literal":"Point"},
        "coordinates":["$longitude","$latitude"]
    }}
},])

I used the above code to fetch data in mongodb and working fine.It gives me the result

 { "location":{
            "type":"Point",
            "coordinates":[77.587073,12.958794] 
        }}

I tried to use the same in golang and it is shown below

pipe :=DB.C("store").Pipe([]bson.M{
        {"$project":bson.M{"location":
        bson.M{"type":
         bson.M{"$literal":"Point"},"coordinates":[]interface{}{"$longitude","$latitude"}}}}}

Above code, throws me an error

panic: bad query: BadValue: Point must be an array or object

so I replaced it like this

pipe :=DB.C("store").Pipe([]bson.M{
        {"$project":bson.M{"location":
        bson.M{"$literal":
         bson.M{"type":"Point"},"coordinates":[]interface{}{"$longitude","$latitude"}}}}})

but this also throws me an error

panic: this object is already an operator expression, and can't be used as a document expression (at 'coordinates')

my complete work is shown in the below link my work is here please help me to solve this. Thank you

Answer 1

To be complete, this is what you actually try to do:

pipe := DB.C("store").Pipe([]bson.M{
    {"$project": bson.M{"location": bson.M{"type": bson.M{"$literal": "Point"}, "coordinates": []interface{}{"$longitude", "$latitude"}}}},
    {"$match": bson.M{"location": bson.M{"$geoWithin": bson.M{"$centerSphere": []interface{}{"$coordinates", 10 / 6378.11}}}}},
})

The problem is not with your "Point" literal, it's just a mere coincidence. If you change it to "Pt" for example, you will still see the exact same error message.

The Point in the error message refers to $centerSphere, which expects a center point and a radius. And the way you try to "pass" it does not work.

This works for example:

"$centerSphere": []interface{}{[]interface{}{1.0, 2.0}, 10 / 6378.11}

Your original query does not make sense, as you try to find documents where the location is within 10 kilometers from itself, which would match all documents.

Instead you want to / should query documents which are within 10 kilometers of a specific location, and you may pass the coordinates of this specific location to $centerSphere:

myLong, myLat := 10.0, 20.0

// ...

"$centerSphere": []interface{}{[]interface{}{myLong, myLat}, 10 / 6378.11}

The complete query:

myLong, myLat := 10.0, 20.0
pipe := DB.C("store").Pipe([]bson.M{
    {"$project": bson.M{"location": bson.M{"type": bson.M{"$literal": "Point"}, "coordinates": []interface{}{"$longitude", "$latitude"}}}},
    {"$match": bson.M{"location.coordinates": bson.M{"$geoWithin": bson.M{"$centerSphere": []interface{}{[]interface{}{myLong, myLat}, 10 / 6378.11}}}}},
})