refactor: prepare to decouple the "model migration" package and "models" package (#38533)

Migrations should never use model structs directly, because the model
structs can be different in different releases. e.g. if one migration uses
"User" model, it works in the early releases, then one day, when the
User model changes, the migration breaks because it will use the
new (incorrect) User model, it should only use the old User model.

The same to "modules/structs".

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

modelmigration/base/db_test.go

@@ -0,0 +1,103 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package base_test
+
+import (
+	"testing"
+
+	"gitea.dev/modelmigration/base"
+	"gitea.dev/modelmigration/migrationtest"
+	"gitea.dev/modules/timeutil"
+
+	"xorm.io/xorm/names"
+)
+
+func TestMain(m *testing.M) {
+	migrationtest.MainTest(m)
+}
+
+func Test_DropTableColumns(t *testing.T) {
+	x, deferable := migrationtest.PrepareTestEnv(t, 0)
+	defer deferable()
+	// FIXME: this logic seems wrong. Need to add an assertion here in the future, but it seems causing failure.
+	if x == nil || t.Failed() {
+		t.Skip("PrepareTestEnv did not yield a usable engine")
+	}
+
+	type DropTest struct {
+		ID            int64 `xorm:"pk autoincr"`
+		FirstColumn   string
+		ToDropColumn  string `xorm:"unique"`
+		AnotherColumn int64
+		CreatedUnix   timeutil.TimeStamp `xorm:"INDEX created"`
+		UpdatedUnix   timeutil.TimeStamp `xorm:"INDEX updated"`
+	}
+
+	columns := []string{
+		"first_column",
+		"to_drop_column",
+		"another_column",
+		"created_unix",
+		"updated_unix",
+	}
+
+	x.SetMapper(names.GonicMapper{})
+
+	for i := range columns {
+		if err := x.Sync(new(DropTest)); err != nil {
+			t.Errorf("unable to create DropTest table: %v", err)
+			return
+		}
+
+		sess := x.NewSession()
+		if err := sess.Begin(); err != nil {
+			sess.Close()
+			t.Errorf("unable to begin transaction: %v", err)
+			return
+		}
+		if err := base.DropTableColumns(sess, "drop_test", columns[i:]...); err != nil {
+			sess.Close()
+			t.Errorf("Unable to drop columns[%d:]: %s from drop_test: %v", i, columns[i:], err)
+			return
+		}
+		if err := sess.Commit(); err != nil {
+			sess.Close()
+			t.Errorf("unable to commit transaction: %v", err)
+			return
+		}
+		sess.Close()
+		if err := x.DropTables(new(DropTest)); err != nil {
+			t.Errorf("unable to drop table: %v", err)
+			return
+		}
+		for j := range columns[i+1:] {
+			if err := x.Sync(new(DropTest)); err != nil {
+				t.Errorf("unable to create DropTest table: %v", err)
+				return
+			}
+			dropcols := append([]string{columns[i]}, columns[j+i+1:]...)
+			sess := x.NewSession()
+			if err := sess.Begin(); err != nil {
+				sess.Close()
+				t.Errorf("unable to begin transaction: %v", err)
+				return
+			}
+			if err := base.DropTableColumns(sess, "drop_test", dropcols...); err != nil {
+				sess.Close()
+				t.Errorf("Unable to drop columns: %s from drop_test: %v", dropcols, err)
+				return
+			}
+			if err := sess.Commit(); err != nil {
+				sess.Close()
+				t.Errorf("unable to commit transaction: %v", err)
+				return
+			}
+			sess.Close()
+			if err := x.DropTables(new(DropTest)); err != nil {
+				t.Errorf("unable to drop table: %v", err)
+				return
+			}
+		}
+	}
+}