how can I findAndModify one record in mongodb with golang?

Question

I want something like this:

1. find one record in mongo db old_record
2. update this record to new_record
3. return old_record

I write code like this:

ret = nil
// First, Find the obj
obj := &orm.QuerySetObj{}
err2 := this.querySetCollection.With(session).Find(objKey).One(obj)
if nil != err2 {
    this.logger.Println("Error find obj")
    return
}

ret = obj

// Then, update this obj
obj.updateTime = time.Now().Unix()
err3 := this.querySetCollection.With(session).Upsert(objKey, obj)
if nil != err3 {
    this.logger.Println("Error update obj")
    return
}

return

but, I think find and update should be an atomic operation, so my code is not safe.

how can I do this in an atomic operation

Answer 1

The method here is .Apply() which takes a Change type and returns ChangeInfo.

Direct example in the documentation:

change := mgo.Change{
        Update: bson.M{"$inc": bson.M{"n": 1}},
        ReturnNew: false,
}
info, err = col.Find(M{"_id": id}).Apply(change, &doc)
fmt.Println(doc.N)

Where doc is document that is found, and it's state depending on the value of ReturnNew in the Change arguments, being false where you want the original document.

Basically all arguments are in the same form as with .findAndModify()