var coll = db.foo; coll.drop(); coll.insert({_id: 1}); assert .neq( null , coll.findOne({_id: 1})); var res = coll.runCommand({compact: coll.getName()}); assert .commandWorked(res, tojson(res)); // this assert fails because findOne returns null assert .neq( null , coll.findOne({_id: 1})); // run validate to see index keys: { "test.foo.$_id_" : 0 } res = coll.runCommand({validate: coll.getName(), full: true }); printjson(res.keysPerIndex);