可以使用下面的聚合

$expr 允许您在其中使用聚合运算符.所以你可以轻松使用 $cond 具有allowedRoles 和没有

$expr allows you use aggregation operator inside it. So you can easily use $cond aggregation for the users who has allowedRoles and who hasn't

db.users.aggregate([
  { "$match": { "name": "Charles" }},
  { "$lookup": {
    "from": "roles",
    "let": { "ar": "$allowedRoles" },
    "pipeline": [
      { "$match": {
        "$expr": {
          "$cond": [
            { "$eq": [{ "$type": "$$ar" }, "missing"] },
            {},
            { "$in": ["$_id", "$$ar"] }
          ]
        }
      }}
    ],
    "as": "roles"
  }}
])