How do I apply an update function to all the documents in a collection in MongoDB (using mgo)?

Question

I've modeled a has-many relationship in MongoDB with a Group that has an array of Students as one of the fields.

When a Student is deleted, I want to go through all the Groups, and for each Group that has the deleted_student in its Group.Students, remove the deleted_student from the array.

To remove the deleted_student from the array, I have a helper function, RemoveItem that I'd liek to use.

What's the "Mongo" way of apply this function to all the records in a collection? Or should I just return all the Groups, then iterate through each and perform the operation on the field. Something like this (in Go using the mgo library)

groups := conn.C("groups")
allGroups := groups.Find()
for _, group := range allGroups {
    group.Students = RemoveItem(id, group.Students)
}

// Helper function that removes a mgo.DBRef from an array of mgo.DBRefs based on ID string
func RemoveItem(objectId string, array []mgo.DBRef) []mgo.DBRef {
    updatedArray := []mgo.DBRef{}
    for _, item := range array {
        if item.Id != objectId {
            updatedArray = append(updatedArray, item)
        }
    }
    return updatedArray
}

Answer 1

I believe it is still not possible to do a single operation to update select elements within an array based on criteria. However, you can limit your results a bit before beginning your iteration.

groupsToBeUpdated := groups.Find(bson.M{"students": bson.M{"$elemMatch": bson.M{"deleted_student": true}}})

From here you can iterate through, however I would inject your db resource into the RemoveItem method so that it can make the update there. This allows you to concurrently update the documents. See my gist here for a worker example: https://gist.github.com/KyleWolfe/7c54018a9ed16517c0dd or on playground: http://play.golang.org/p/njtAALaH1t

Edit: Additionally, if you had an interface set up that also held your db resource, you could then have a method of RemoveStudents(). Everything that method would need would be accessible though your Group struct.

Answer 2

await schema.updateMany(
    { },
    [
       { $set: {  toupdate:  "$name" } }    ]
 );