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/v1_15/v182_test.go

@@ -0,0 +1,60 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_15
+
+import (
+	"testing"
+
+	"gitea.dev/modelmigration/migrationtest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_AddIssueResourceIndexTable(t *testing.T) {
+	// Create the models used in the migration
+	type Issue struct {
+		ID     int64 `xorm:"pk autoincr"`
+		RepoID int64 `xorm:"UNIQUE(s)"`
+		Index  int64 `xorm:"UNIQUE(s)"`
+	}
+
+	// Prepare and load the testing database
+	x, deferable := migrationtest.PrepareTestEnv(t, 0, new(Issue))
+	if x == nil || t.Failed() {
+		defer deferable()
+		return
+	}
+	defer deferable()
+
+	// Run the migration
+	if err := AddIssueResourceIndexTable(x); err != nil {
+		assert.NoError(t, err)
+		return
+	}
+
+	type ResourceIndex struct {
+		GroupID  int64 `xorm:"pk"`
+		MaxIndex int64 `xorm:"index"`
+	}
+
+	start := 0
+	const batchSize = 1000
+	for {
+		indexes := make([]ResourceIndex, 0, batchSize)
+		err := x.Table("issue_index").Limit(batchSize, start).Find(&indexes)
+		assert.NoError(t, err)
+
+		for _, idx := range indexes {
+			var maxIndex int
+			has, err := x.SQL("SELECT max(`index`) FROM issue WHERE repo_id = ?", idx.GroupID).Get(&maxIndex)
+			assert.NoError(t, err)
+			assert.True(t, has)
+			assert.EqualValues(t, maxIndex, idx.MaxIndex)
+		}
+		if len(indexes) < batchSize {
+			break
+		}
+		start += len(indexes)
+	}
+}