Mongodb 根据 _id 从另一个集合中获取元素值

Question

I'm trying to get an element value based on a result _id in an aggregation.

This is the aggregation:

$project: {
  _id: 0,
  brand: "$_id",
  "options": {
    $mergeObjects: "$ram"
  },
  sum: {
    $add: [
      "$sm",
      1
    ]
  }
}

And I want brand to get the name from other collection named "brands" which looks like this

    [
  {
    "_id": ObjectId("617b0dbacda6cbd1a0403f68"),
    "SerialNumber": "45454234324",
    "name": "hp"
  },
  {
    "_id": ObjectId("617b0dbacda6cbd1a0403f69"),
    "SerialNumber": "azazz5245454az",
    "name": "asus"
  }]

What I want to get is the name of brand using using its _id based on the result _id. Using SQL its something like this:

Get brands.name where _id=brands._id

Answer 1

i managed to do it using $lookup

{
                $lookup:
                {
                    from: "brands",
                    localField: "brand",
                    foreignField: "_id",
                    as: "brand"
                }
            },
            {
                $set: {
                    brand: "$brand.brand"     
                }
            },