db={"new_users":[{"_id":ObjectId("615a1c0e38df3c1c6a5a0c01"),"firstName":"John","lastName":"Doe"},{"_id":ObjectId("615a1c0e38df3c1c6a5a0c02"),"firstName":"Jane","lastName":"Smith"}],"workspaces":[{"_id":ObjectId("615a1c0e38df3c1c6a5a0c11"),"name":"Workspace A","users":[{"groups":[],"workSpaceRole":"Admin","userID":"615a1c0e38df3c1c6a5a0c01"},{"groups":[],"workSpaceRole":"Member","userID":"615a1c0e38df3c1c6a5a0c02"}]},{"_id":ObjectId("615a1c0e38df3c1c6a5a0c12"),"name":"Workspace B","users":[{"groups":[],"workSpaceRole":"Member","userID":"615a1c0e38df3c1c6a5a0c02"}]}]}

db.workspaces.aggregate([{$unwind:"$users"},{$addFields:{"users.userID":{$toObjectId:"$users.userID"}}},{$lookup:{from:"new_users",localField:"users.userID",foreignField:"_id",as:"userData"}}])