handle empty set, add test

src/database/mongo/sorted.js

@@ -36,6 +36,9 @@ module.exports = function (db, module) {
 		}
 
 		if (Array.isArray(key)) {
+			if (!key.length) {
+				return setImmediate(callback, null, []);
+			}
 			key = { $in: key };
 		}
 

src/topics/suggested.js

@@ -4,6 +4,7 @@
 var async = require('async');
 var _ = require('lodash');
 
+const db = require('../database');
 var privileges = require('../privileges');
 var search = require('../search');
 
@@ -46,15 +47,10 @@ module.exports = function (Topics) {
 				Topics.getTopicTags(tid, next);
 			},
 			function (tags, next) {
-				async.map(tags, function (tag, next) {
+				db.getSortedSetRevRange(tags.map(tag => 'tag:' + tag + ':topics'), 0, -1, next);
-					Topics.getTagTids(tag, 0, -1, next);
-				}, next);
-			},
-			function (data, next) {
-				next(null, _.uniq(_.flatten(data)));
 			},
 			function (tids, next) {
-				tids = tids.map(Number);
+				tids = _.uniq(tids).map(Number);
 				privileges.topics.filterTids('read', tids, uid, next);
 			},
 		], callback);

test/database/sorted.js

@@ -139,6 +139,14 @@ describe('Sorted Set methods', function () {
 				done();
 			});
 		});
+
+		it('should return empty array if keys is empty array', function (done) {
+			db.getSortedSetRange([], 0, -1, function (err, data) {
+				assert.ifError(err);
+				assert.deepStrictEqual(data, []);
+				done();
+			});
+		});
 	});
 
 	describe('getSortedSetRevRange()', function () {