Aggregate multiple lookups return no data-CodePudding

I have documents like this in DB. And I need to grab the data of each item based on the itemType from their own collection.

      { listId: 2, itemType: 'book', itemId: 5364 },
      { listId: 2, itemType: 'car', itemId: 354 },
      { listId: 2, itemType: 'laptop', itemId: 228 }

Based on MongoDB docs and some search, I figured out that I need to use let and $expr in lookup, to make some condition.

ListItemsModel.aggregate([
    { $match: { listId: 2 } },
    { $lookup:
       {
         from: 'books',
         localField: 'itemId',
         foreignField: '_id',
         let: { "itemType": "$itemType" },
         pipeline: [
            { $project: { _id: 1, title: 1 }},
            { $match: { $expr: { $eq: ["$$itemType", "book"] } }}
         ],
         as: 'data'
       }
     },
     { $lookup:
        {
          from: 'cars',
          localField: 'itemId',
          foreignField: '_id',
          let: { "itemType": "$itemType" },
          pipeline: [
             { $project: { _id: 1, title: 1 }},
             { $match: { $expr: { $eq: ["$$itemType", "car"] } }}
          ],
          as: 'data'
        }
      },
      { $lookup:
       {
         from: 'laptops',
         localField: 'itemId',
         foreignField: '_id',
         let: { "itemType": "$itemType" },
         pipeline: [
            { $project: { _id: 1, title: 1 }},
            { $match: { $expr: { $eq: ["$$itemType", "laptop"] } }}
         ],
         as: 'data'
       }
     }
    ]);

The problem is, in the result all data fields are empty as data: []. The syntax seems correct to me. What's wrong?

CodePudding user response：

Any subsequent reassignment of field values will eliminate any previous value.

So, for your aggregation pipeline, you need to assign different values to each "$lookup" "as" field.

For example:

     // ...
     { $lookup:
       {
         from: 'books',
         // ...
         as: 'booksData'
       }
     },
     { $lookup:
       {
         from: 'cars',
         // ...
         as: 'carsData'
       }
     },
     { $lookup:
       {
         from: 'laptops',
         // ...
         as: 'laptopsData'
       }
     },
     // ...