[PORT] gitea#30190: Add unique index for project_issue to prevent duplicate data

Fix #27639

---

Remove MSSQL specific code per https://codeberg.org/forgejo/discussions/issues/122

(cherry picked from commit b4825670596fe745cebdcc63a8ead4388602d42c)

models/migrations/v1_23/v294_test.go

@@ -0,0 +1,52 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23 //nolint
+
+import (
+	"slices"
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"github.com/stretchr/testify/assert"
+	"xorm.io/xorm/schemas"
+)
+
+func Test_AddUniqueIndexForProjectIssue(t *testing.T) {
+	type ProjectIssue struct { //revive:disable-line:exported
+		ID        int64 `xorm:"pk autoincr"`
+		IssueID   int64 `xorm:"INDEX"`
+		ProjectID int64 `xorm:"INDEX"`
+	}
+
+	// Prepare and load the testing database
+	x, deferable := base.PrepareTestEnv(t, 0, new(ProjectIssue))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	cnt, err := x.Table("project_issue").Where("project_id=1 AND issue_id=1").Count()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 2, cnt)
+
+	assert.NoError(t, AddUniqueIndexForProjectIssue(x))
+
+	cnt, err = x.Table("project_issue").Where("project_id=1 AND issue_id=1").Count()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, cnt)
+
+	tables, err := x.DBMetas()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, len(tables))
+	found := false
+	for _, index := range tables[0].Indexes {
+		if index.Type == schemas.UniqueType {
+			found = true
+			slices.Equal(index.Cols, []string{"project_id", "issue_id"})
+			break
+		}
+	}
+	assert.True(t, found)
+}