How to replace a document in MongoDB with Go driver? - mongodb

Question

I'm trying to update a document in MongoDB with mongodb/mongo-go-driver. From its doc, one document can be replaced with:
var coll *mongo.Collection
var id primitive.ObjectID
// find the document for which the _id field matches id and add a field called "location"
// specify the Upsert option to insert a new document if a document matching the filter isn't found
opts := options.Replace().SetUpsert(true)
filter := bson.D{{"_id", id}}
replacement := bson.D{{"location", "NYC"}}
result, err := coll.ReplaceOne(context.TODO(), filter, replacement, opts)
if err != nil {
log.Fatal(err)
}
if result.MatchedCount != 0 {
fmt.Println("matched and replaced an existing document")
return
}
if result.UpsertedCount != 0 {
fmt.Printf("inserted a new document with ID %v\n", result.UpsertedID)
}
But what if I have fields more than like 20. I am wondering if I can update without re-writing all fields again, I tried something like this:
// Replacement struct
type Input struct {
Name *string `json:"name,omitempty" bson:"name,omitempty" validate:"required"`
Description *string `json:"description,omitempty" bson:"description,omitempty"`
Location *string `json:"location,omitempty" bson:"location,omitempty"`
}
...
oid, _ := primitive.ObjectIDFromHex(id) // because 'id' from request is string
filter := bson.M{"_id", oid} // throws `[compiler] [E] missing key in map literal`
// ^^^
replacement := bson.D{{"$set", input}} // throws `composite literal uses unkeyed fields`
// ^^^^^
result, err := coll.ReplaceOne(context.TODO(), filter, replacement, opts)
...
But it throws errors at filter and replacement. How can I replace a whole document properly?

Answer 1

bson.M is a map, so if you use it, you have to write: bson.M{"_id": oid}. See missing type in composite literal go AND missing key in map literal go.
And bson.D is a slice of structs, so if you use it, you should write bson.D{{Key:"$set", Value: input}}. Omitting the field names is not a compiler error, it's just a go vet warning.
Now on to replacing. The replacement must be a document itself, without using $set (this is not updating but replacing). For reference see MongoDB's collection.replaceOne() and the driver's doc: Collection.ReplaceOne():
The replacement parameter must be a document that will be used to replace the selected document. It cannot be nil and cannot contain any update operators (https://docs.mongodb.com/manual/reference/operator/update/).
So do it like this:
filter := bson.M{"_id": oid}
result, err := coll.ReplaceOne(context.TODO(), filter, input, opts)

Answer 2

Here is the complete code. This will help readers to understand the code flow.
func UpdateFunction(echoCtx echo.Context) (error) {
//Web Section
documentID := echoCtx.Param("id") //Provided in URL
var newObject myStruct
err := echoCtx.Bind(&newObject) // Json with updated values sent by web client mapped to struct
ctx := echoCtx.Request().Context()
//Database Section
database := db.Conn.Database("myDatabase")
collection := database.Collection("myCollection")
existingHexID, err := primitive.ObjectIDFromHex(documentID)
if err != nil {
fmt.Println("ObjectIDFromHex ERROR", err)
} else {
fmt.Println("ObjectIDFromHex:", existingHexID)
}
// Replacing OLD document with new using _id
filter := bson.M{"_id": newDocumentID}
result, err := collection.ReplaceOne(ctx, filter, newObject)
if err != nil {
log.Fatal(err)
}
fmt.Printf(
"insert: %d, updated: %d, deleted: %d /n",
result.MatchedCount,
result.ModifiedCount,
result.UpsertedCount,
)
return echoCtx.JSON(http.StatusOK, result.ModifiedCount)
}