Resmoke test: (function() { const st = new ShardingTest({ shards: 2, mongos: 1, other: { mongosOptions: {setParameter: {mrEnableSingleReduceOptimization: true }}, shardOptions: {setParameter: {mrEnableSingleReduceOptimization: true }}, } }); const mongosDB = st.s0.getDB(jsTestName()); const mongosColl = mongosDB[jsTestName()]; assert .commandWorked(mongosDB.dropDatabase()); assert .commandWorked(mongosDB.adminCommand({enableSharding: mongosDB.getName()})); st.ensurePrimaryShard(mongosDB.getName(), st.shard0.shardName); assert .commandWorked( mongosDB.adminCommand({shardCollection: mongosColl.getFullName(), key: {_id: 1}})); // Split the collection into 4 chunks: [MinKey, -100), [-100, 0), [0, 100), [100, MaxKey). assert .commandWorked(mongosDB.adminCommand({split: mongosColl.getFullName(), middle: {_id: 0}})); // Move the [0, 100) and [100, MaxKey) chunks to shard1. assert .commandWorked(mongosDB.adminCommand( {moveChunk: mongosColl.getFullName(), find: {_id: 50}, to: st.shard1.shardName})); assert .commandWorked(mongosDB.adminCommand( {moveChunk: mongosColl.getFullName(), find: {_id: 150}, to: st.shard1.shardName})); assert .commandWorked(mongosColl.insert({_id: -1})); const map = function() { emit(0, {val: "mapped value" }); }; const reduce = function(key, values) { return {val: "reduced value" }; }; let res = assert .commandWorked(mongosDB.runCommand( {mapReduce: mongosColl.getName(), map: map, reduce: reduce, out: {inline: 1}})); assert .eq(res.results[0], {_id: 0, value: {val: "mapped value" }}); assert .commandWorked(mongosColl.insert({_id: 1})); res = assert .commandWorked(mongosDB.runCommand( {mapReduce: mongosColl.getName(), map: map, reduce: reduce, out: {inline: 1}})); assert .eq(res.results[0], {_id: 0, value: {val: "reduced value" }}); st.stop(); }());