fix: #7921, dont create wrong entries in db for flag

upgrade script to remove wrong entries

src/flags.js

@@ -445,7 +445,7 @@ Flags.getTargetCid = async function (type, id) {
 	if (type === 'post') {
 		return await posts.getCidByPid(id);
 	}
-	return id;
+	return null;
 };
 
 Flags.update = function (flagId, uid, changeset, callback) {

src/upgrades/1.13.0/clean_flag_byCid.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const db = require('../../database');
+const batch = require('../../batch');
+
+module.exports = {
+	name: 'Clean flag byCid zsets',
+	timestamp: Date.UTC(2019, 8, 24),
+	method: async function (callback) {
+		const progress = this.progress;
+
+		await batch.processSortedSet('flags:datetime', async function (flagIds) {
+			progress.incr(flagIds.length);
+			const flagData = await db.getObjects(flagIds.map(id => 'flag:' + id));
+			const bulkRemove = [];
+			for (const flagObj of flagData) {
+				if (flagObj && flagObj.type === 'user' && flagObj.targetId && flagObj.flagId) {
+					bulkRemove.push(['flags:byCid:' + flagObj.targetId, flagObj.flagId]);
+				}
+			}
+
+			await db.sortedSetRemoveBulk(bulkRemove);
+		}, {
+			progress: progress,
+		});
+		callback();
+	},
+};