$project in $lookup pipeline does not avoid loading the whole document

Working with Data
,
2

Hi Cedric,

The syntax to use lookup with pipeline is slightly different as mentioned in the docs - Lookup

I doubt the time consumption is due to the mentioned veryHeavyField but still i think the below modified query may solve your purpose as it in the beginning itself removes the veryHeavyField.

[
  {
    $match: {
      isDeleted: false
    },
  },
  {
    $lookup: {
      from: "joined",
      let: {
        id: "$_id"
      }
      pipeline: [
        {
          $project: { veryHeavyField: 0 }
        },
        {
          $match: {
            $expr: {
              $eq: ["$$id", "$sourceId"]
            }
          }
        }
      ]
      as: "joined",
    },
  }
]

Hope this helps!!