diff --git a/common/utils.go b/common/utils.go
new file mode 100644
index 0000000..df35d10
--- /dev/null
+++ b/common/utils.go
@@ -0,0 +1,11 @@
+package common
+
+import (
+	"strings"
+)
+
+// SafeUsername makes a string lowercase and replaces all spaces with
+// underscores.
+func SafeUsername(s string) string {
+	return strings.Replace(strings.ToLower(s), " ", "_", -1)
+}
diff --git a/common/utils_test.go b/common/utils_test.go
new file mode 100644
index 0000000..2769502
--- /dev/null
+++ b/common/utils_test.go
@@ -0,0 +1,20 @@
+package common
+
+import "testing"
+
+func TestSafeUsername(t *testing.T) {
+	tests := []struct {
+		name string
+		arg  string
+		want string
+	}{
+		{"noChange", "no_change", "no_change"},
+		{"toLower", "Change_Me", "change_me"},
+		{"complete", "La_M a m m a_putt na", "la_m_a_m_m_a_putt_na"},
+	}
+	for _, tt := range tests {
+		if got := SafeUsername(tt.arg); got != tt.want {
+			t.Errorf("%q. SafeUsername() = %v, want %v", tt.name, got, tt.want)
+		}
+	}
+}
diff --git a/common/where_test.go b/common/where_test.go
index 4dbfc2e..144f05f 100644
--- a/common/where_test.go
+++ b/common/where_test.go
@@ -29,7 +29,7 @@ func Test_generateQuestionMarks(t *testing.T) {
 func TestWhereClause_In(t *testing.T) {
 	type args struct {
 		initial string
-		fields  []interface{}
+		fields  []string
 	}
 	tests := []struct {
 		name   string
@@ -40,13 +40,13 @@ func TestWhereClause_In(t *testing.T) {
 		{
 			"simple",
 			&WhereClause{},
-			args{"users.id", []interface{}{"1", "2", "3"}},
+			args{"users.id", []string{"1", "2", "3"}},
 			&WhereClause{"WHERE users.id IN (?, ?, ?)", []interface{}{"1", "2", "3"}},
 		},
 		{
 			"withExisting",
 			Where("users.username = ?", "Howl").Where("users.xd > ?", "6"),
-			args{"users.id", []interface{}{"1"}},
+			args{"users.id", []string{"1"}},
 			&WhereClause{
 				"WHERE users.username = ? AND users.xd > ? AND users.id IN (?)",
 				[]interface{}{"Howl", "6", "1"},