diff --git a/models/migrations/migrations.go b/models/migrations/migrations.go
index 9b94c49cc06..565e583156b 100644
--- a/models/migrations/migrations.go
+++ b/models/migrations/migrations.go
@@ -233,6 +233,8 @@ var migrations = []Migration{
 	NewMigration("remove issue dependency comments who refer to non existing issues", purgeInvalidDependenciesComments),
 	// v149 -> v150
 	NewMigration("Add Created and Updated to Milestone table", addCreatedAndUpdatedToMilestones),
+	// v150 -> v151
+	NewMigration("add primary key to repo_topic", addPrimaryKeyToRepoTopic),
 }
 
 // GetCurrentDBVersion returns the current db version
diff --git a/models/migrations/v150.go b/models/migrations/v150.go
new file mode 100644
index 00000000000..41d6a90401a
--- /dev/null
+++ b/models/migrations/v150.go
@@ -0,0 +1,39 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+func addPrimaryKeyToRepoTopic(x *xorm.Engine) error {
+	// Topic represents a topic of repositories
+	type Topic struct {
+		ID          int64  `xorm:"pk autoincr"`
+		Name        string `xorm:"UNIQUE VARCHAR(25)"`
+		RepoCount   int
+		CreatedUnix timeutil.TimeStamp `xorm:"INDEX created"`
+		UpdatedUnix timeutil.TimeStamp `xorm:"INDEX updated"`
+	}
+
+	// RepoTopic represents associated repositories and topics
+	type RepoTopic struct {
+		RepoID  int64 `xorm:"pk"`
+		TopicID int64 `xorm:"pk"`
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	recreateTable(sess, &Topic{})
+	recreateTable(sess, &RepoTopic{})
+
+	return sess.Commit()
+}
diff --git a/models/topic.go b/models/topic.go
index 4a5bffa08a5..0075c1702de 100644
--- a/models/topic.go
+++ b/models/topic.go
@@ -25,7 +25,7 @@ var topicPattern = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
 
 // Topic represents a topic of repositories
 type Topic struct {
-	ID          int64
+	ID          int64  `xorm:"pk autoincr"`
 	Name        string `xorm:"UNIQUE VARCHAR(25)"`
 	RepoCount   int
 	CreatedUnix timeutil.TimeStamp `xorm:"INDEX created"`
@@ -34,8 +34,8 @@ type Topic struct {
 
 // RepoTopic represents associated repositories and topics
 type RepoTopic struct {
-	RepoID  int64 `xorm:"UNIQUE(s)"`
-	TopicID int64 `xorm:"UNIQUE(s)"`
+	RepoID  int64 `xorm:"pk"`
+	TopicID int64 `xorm:"pk"`
 }
 
 // ErrTopicNotExist represents an error that a topic is not exist