Fix SQL errors for Timeline view,

when using Postgres or Sqlite. Closes #194

api/database/helpers.go

@@ -0,0 +1,48 @@
+package database
+
+import (
+	"fmt"
+	"log"
+
+	"gorm.io/gorm"
+)
+
+// DateComponent a component of a date (day, month, year)
+type DateComponent string
+
+const (
+	DateCompYear  DateComponent = "YEAR"
+	DateCompMonth DateComponent = "MONTH"
+	DateCompDay   DateComponent = "DAY"
+)
+
+// DateExtract is a helper function that is used to generate the proper SQL syntax
+// for extracting date components (day, month, year) for different database backends.
+func DateExtract(db *gorm.DB, component DateComponent, attribute string) string {
+
+	var result string
+
+	switch db.Dialector.Name() {
+	case "mysql", "postgress":
+		result = fmt.Sprintf("EXTRACT(%s FROM %s)", component, attribute)
+		break
+	case "sqlite":
+
+		var sqliteFormatted string
+		switch component {
+		case DateCompYear:
+			sqliteFormatted = "%Y"
+		case DateCompMonth:
+			sqliteFormatted = "%m"
+		case DateCompDay:
+			sqliteFormatted = "%d"
+		}
+
+		result = fmt.Sprintf("CAST(strftime('%s', %s) AS INTEGER)", sqliteFormatted, attribute)
+		break
+	default:
+		log.Panicf("unsupported database backend: %s", db.Dialector.Name())
+	}
+
+	return result
+}