Conditional $lookup in MongoDB?

Question

I have two collections in MongoDB 3.6:

users: [
  {name: "John", allowedRoles: [1, 2, 3]},
  {name: "Charles", allowedRoles: [1]},
  {name: "Sarah", isAdmin: true}
]

roles: [
  {_id: 1, name: "Foo", description: "This role allows to foo the blargs"},
  {_id: 2, name: "Bar", description: "..."},
  {_id: 3, name: "Doh", descripcion: "..."}
]

I'm very new to MongoDB; I just figured out how to query an user and join all the data from his roles, using the $lookup aggregation stage:

db.users.aggregate([{
  "$match": { "name": "John" }          // Or without this $match part, to get all users
},{                                     //
  "$lookup": {
    "from": "roles",
    "localField": "allowedRoles",
    "foreignField": "_id",
    "as": "roles"
  }
}]);

It works for my regular users, who have an array of allowed roles IDs. I have also administrator users, which can access all existing roles, but don't have the allowedRoles array (it would be a burden to maintain, since new roles will be created frequently). So, instead of specifying the join fields, I do a $lookup with an empty pipeline, to get the cartesian product of both collections:

db.users.aggregate([{
  "$match": { "name": "Sarah" }
},{
  "$lookup": {
    "from": "roles",
    "pipeline": [],
    "as": "roles"
  }
}]);

Is there any way to have both, with a single query? With a conditional expression or something?

In an SQL database, I would simply include the condition in the join:

select users.*, roles.*
from users
left join users_x_roles inter on users.id = inter.user_id
left join roles on inter.role_id = roles.id or users.is_admin = 1;
--                                          ^^^^^^^^^^^^^^^^^^^^^

Answer 1

You can use below aggregation

$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"
  }}
])