fix: make sure null values are always sorted last across all dmbs

pkg/models/task_collection_test.go

@@ -1046,6 +1046,9 @@ func TestTaskCollection_ReadAll(t *testing.T) {
 				a: &user.User{ID: 1},
 			},
 			want: []*Task{
+				// The only tasks with a position set
+				task1,
+				task2,
 				// the other ones don't have a position set
 				task3,
 				task4,
@@ -1076,9 +1079,51 @@ func TestTaskCollection_ReadAll(t *testing.T) {
 				task31,
 				task32,
 				task33,
-				// The only tasks with a position set
-				task1,
+			},
+		},
+		{
+			name: "order by due date",
+			fields: fields{
+				SortBy:  []string{"due_date", "id"},
+				OrderBy: []string{"asc", "desc"},
+			},
+			args: args{
+				a: &user.User{ID: 1},
+			},
+			want: []*Task{
+				// The only tasks with a due date
+				task6,
+				task5,
+				// The other ones don't have a due date
+				task33,
+				task32,
+				task31,
+				task30,
+				task29,
+				task28,
+				task27,
+				task26,
+				task25,
+				task24,
+				task23,
+				task22,
+				task21,
+				task20,
+				task19,
+				task18,
+				task17,
+				task16,
+				task15,
+				task12,
+				task11,
+				task10,
+				task9,
+				task8,
+				task7,
+				task4,
+				task3,
 				task2,
+				task1,
 			},
 		},
 		{

pkg/models/tasks.go

@@ -296,17 +296,30 @@ func getRawTasksForLists(s *xorm.Session, lists []*List, a web.Auth, opts *taskO
 		if err := param.validate(); err != nil {
 			return nil, 0, 0, err
 		}
-		orderby += param.sortBy + " " + param.orderBy.String()
 
-		// Postgres sorts by default entries with null values after ones with values.
-		// To make that consistent with the sort order we have and other dbms, we're adding a separate clause here.
-		if db.Type() == schemas.POSTGRES {
+		// Mysql sorts columns with null values before ones without null value.
+		// Because it does not have support for NULLS FIRST or NULLS LAST we work around this by
+		// first sorting for null (or not null) values and then the order we actually want to.
+		if db.Type() == schemas.MYSQL {
 			if param.orderBy == orderAscending {
-				orderby += " NULLS FIRST"
+				orderby += param.sortBy + " IS NULL, "
 			}
 			if param.orderBy == orderDescending {
+				orderby += param.sortBy + " IS NOT NULL, "
+			}
+		}
+
+		orderby += param.sortBy + " " + param.orderBy.String()
+
+		// Postgres and sqlite allow us to control how columns with null values are sorted.
+		// To make that consistent with the sort order we have and other dbms, we're adding a separate clause here.
+		if db.Type() == schemas.POSTGRES || db.Type() == schemas.SQLITE {
+			if param.orderBy == orderAscending {
 				orderby += " NULLS LAST"
 			}
+			if param.orderBy == orderDescending {
+				orderby += " NULLS FIRST"
+			}
 		}
 
 		if (i + 1) < len(opts.sortby) {