Project field from array inside document

Question

Suppose you have the following documents in my collection:

{
    "_id" : ObjectId("5ddb10ff6e25e6fec648df70"),
    "shapes" : [
        {
            "color" : [
                "blue",
                "red"
            ],
            "shape" : "square"
        },
        {
            "color" : [
                "red",
                "green"
            ],
            "shape" : "circle"
        }
    ],
    "name" : "customer1"
}

I have the following query:

db.customers
  .aggregate([
    { $match: { "shapes.color": "red" } },
    {
      $project: {
        shapes: {
          $filter: {
            input: "$shapes",
            as: "shape",
            cond: { $eq: ["$$shape.color", "red"] }
          }
        }
      }
    }
  ])
  .pretty();

Returns matched document:

{ "_id" : ObjectId("5ddb10ff6e25e6fec648df70"), "shapes" : [ ] }

But I was expecting this:

{
    "_id" : ObjectId("5ddb10629f3f949e3feddf19"),
    "shapes" : [
        {
            "color" : ["red"],
            "shape" : "square"
        },
        {
            "color" : ["red"],
            "shape" : "circle"
        }
    ]
}

How do I accomplish this?

Ashh · Accepted Answer · 2019-11-25 05:03:20Z

You need to use $filter two times as you have two double nested arrays here

db.collection.aggregate([
  { "$match": { "shapes.color": "red" } },
  { "$project": {
    "shapes": {
      "$filter": {
        "input": {
          "$map": {
            "input": "$shapes",
            "in": {
              "shape": "$$this.shape",
              "color": {
                "$filter": {
                  "input": "$$this.color",
                  "cond": { "$eq": ["$$this", "red"] }
                }
              }
            }
          }
        },
        "as": "shape",
        "cond": { "$ne": ["$$shape.color", []] }
      }
    }
  }}
])

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