db={"users":[{"_id":"5e6a293637a3d101b488278d","username":"Mabel","role":"user","__v":0},{"_id":"5e6a294537a3d101b488278e","username":"Tom","role":"user","__v":0},{"_id":"5e6a294f37a3d101b488278f","username":"Bob","role":"user","__v":0}],"projects":[{"_id":"5e6a298537a3d101b4882790","title":"My Post","description":"First post","userId":"5e6a293637a3d101b488278d","__v":0},{"_id":"5e6a29a237a3d101b4882791","title":"My Post 2","description":"Second post","userId":"5e6a293637a3d101b488278d","__v":0},{"_id":"5e6a29bd37a3d101b4882792","title":"Tom post","description":"Tom post description","userId":"5e6a294537a3d101b488278e","__v":0},{"_id":"5e6a2a0837a3d101b4882793","title":"Bob post","description":"Bob post description","userId":"5e6a294f37a3d101b488278f","__v":0}]}

db.users.aggregate([{$match:{"username":"Mabel"}},{$lookup:{from:"projects",localField:"_id",foreignField:"userId",as:"projects"}}])