transformer: add transformer for unsetting empty values

example_test.go

@@ -107,17 +107,30 @@ func basicCreateAndPopulateKeyspace(t *testing.T, session gocqlx.Session) {
 
 	// Insert song using query builder.
 	insertSong := qb.Insert("examples.songs").
-		Columns("id", "title", "album", "artist", "tags", "data").Query(session)
+		Columns("id", "title", "album", "artist", "tags", "data")
 
-	insertSong.BindStruct(Song{
+	insertQuery := insertSong.Query(session).BindStruct(Song{
 		ID:     mustParseUUID("756716f7-2e54-4715-9f00-91dcbea6cf50"),
 		Title:  "La Petite Tonkinoise",
 		Album:  "Bye Bye Blackbird",
 		Artist: "Joséphine Baker",
 		Tags:   []string{"jazz", "2013"},
 		Data:   []byte("music"),
 	})
-	if err := insertSong.ExecRelease(); err != nil {
+	if err := insertQuery.ExecRelease(); err != nil {
+		t.Fatal("ExecRelease() failed:", err)
+	}
+
+	// Insert song with some empty fields using the same statement.
+	insertQuery = insertSong.Query(session).
+		WithBindTransformer(gocqlx.UnsetEmptyTransformer).
+		BindStruct(Song{
+			ID:     mustParseUUID("94db5bd8-ca97-4d50-9832-14d583c832cd"),
+			Title:  "Moulin Rouge",
+			Artist: "Joséphine Baker",
+			Data:   []byte("music"),
+		})
+	if err := insertQuery.ExecRelease(); err != nil {
 		t.Fatal("ExecRelease() failed:", err)
 	}
 

transformer.go

@@ -4,10 +4,27 @@
 
 package gocqlx
 
+import (
+	"reflect"
+
+	"github.com/gocql/gocql"
+)
+
 // Transformer transforms the value of the named parameter to another value.
 type Transformer func(name string, val interface{}) interface{}
 
 // DefaultBindTransformer just do nothing.
 //
 // A custom transformer can always be set per Query.
 var DefaultBindTransformer Transformer
+
+// UnsetEmptyTransformer unsets all empty parameters.
+// It helps to avoid tombstones when using the same insert/update
+// statement for filled and partially filled named parameters.
+var UnsetEmptyTransformer = func(name string, val interface{}) interface{} {
+	v := reflect.ValueOf(val)
+	if v.IsZero() {
+		return gocql.UnsetValue
+	}
+	return val
+}