Update query in MGO driver, works with bson.M, but does not work with custome structure

Question

Mgo and golang question.

I run into problem again. I try to update record in the database, but running simple command visitors.UpdateId(v.Id, bson.M{"$set": zscore}); where zscore is a variable of type Zscore, does not work. However if I manually convert zscore to bson.M structure, everything works fine.

Does anybody know how to update the record in mongodb using mgo, without manually dumping structure values into bson.M?

Example:

type Zscore struct {
    a float64 `bson:"a,omitempty" json:"a"`
    b float64 `bson:"b,omitempty" json:"b"`
    c float64 `bson:"c,omitempty" json:"c"`
}

v := Visitor{}
zscore := Zscore{}

visitors := updater.C("visitors")

for result.Next(&v) {
    zscore.a = 1
    zscore.b = 2
    zscore.c = 0

    //does not work
    if err := visitors.UpdateId(v.Id, bson.M{"$set": zscore}); err != nil    {
            log.Printf("Got error while updating visitor: %v\n", err)
    }

    //works
    set := bson.M{
        "zscore.a": zscore.a,
        "zscore.b": zscore.b,
        "zscore.c": zscore.c,
    }

    if err := visitors.UpdateId(v.Id, bson.M{"$set": set}); err != nil {
        log.Printf("Got error while updating visitor: %v\n", err)
    }
}

Answer 1

All Go marshaling packages I'm aware of, including the bson package, will not marshal fields that are private (start with a lowercase letter). To fix the issue, just export the relevant fields by uppercasing the first letter of their name.

Also note that, besides the issue mentioned above, the first part of your example will not marshal in an equivalent way to the second part. bson.M{"$set": zscore} is equivalent to bson.M{"$set": bson.M{"a": ... etc ...}}.