Mongo 3.2使用$ group进行$ lookup

Question

我在mongo 3.2中具有以下文档结构

server.start(
  {
    port: 4444,
    cors: {
      credentials: true,
      origin: process.env.FRONTEND_URL
    },
    deets: deets => {
      console.log(`Server is now running on localhost:${deets.port}`);
    }
  },
  options
);

我正在使用$ lookup和$ group来从角色集合中获取实际的角色_id和名称。

{
"_id" : ObjectId("5c16b2e5ef0616776e63b401"),
"brands" : [ ],
"vendors" : [ ],
"categories" : [ ],
"roles" : [
    ObjectId("5c16b2e4ef0616776e63b400")
],
"userId" : ObjectId("5bf2e384186cee7f5b42e776"),
"tenantId" : "2100",
"status" : "Active",
"__v" : 0
}

我使用的查询，但是{ "_id" : ObjectId("5c10145b385a1c52ba0494c9"), "permissions" : [ "orders:view", "orders:export", "order:view", "order:edit" ], "name" : "Orders Management", "tenantId" : "1400", "__v" : 2 } 数组总是为空，我在做什么错？

roles