mgo.v2，$ sample始终返回1条相同的记录

Question

I need get 1 rand record from a collection. I check it on robo-mongo and it work great. But in golang project using mgo.v2, It always return the same record all the time . Is there any bug with the library, or I misunderstanding?

I use "gopkg.in/mgo.v2". Here is my code:

var err error

    pipe := col.Pipe([]bson.M{
        {"$match": bson.M{ "ad_group_id": worker.creative.AdGroupId }},
        {"$sample": bson.M{ "size": 1 }},
    })
    var resp []model.Coordinate
    err = pipe.All(&resp)

log.Print("++++++++++++++++++++++++++++++++++++++++++++++++++: ",resp)

=>

[
    {
        ObjectIdHex("...") ObjectIdHex("...") ObjectIdHex("...") 
        {
            Point [33 33]
        } 
        t3
    }
]

Coordinate collections:

Answer 1

Your use of the mgo package is invalid. If you use Pipeline.One(), you should pass a target of a single model.Coordinate or *model.Coordinate:

var resp model.Coordinate
err = pipe.One(&resp)

Otherwise you get an error like:

Unsupported document type for unmarshalling: []model.Coordinate

If you're going to get a single result, then of course there is no need to specify a size > 1 in the pipeline, so simply use:

{"$sample": bson.M{"size": 1}},

If you want to get multiple random results, then you may use a slice and Pipeline.All() like in this example:

pipe := col.Pipe([]bson.M{
    {"$match": bson.M{"ad_group_id": worker.creative.AdGroupId}},
    {"$sample": bson.M{"size": 2}},
})
var resp []model.Coordinate
err = pipe.All(&resp)