MongoDB aggregation pipeline to modify objects in array in an smart way

Question

I have a MongoDB collection which documents have an array of objects. Like this:

{
  _id: ...,
  a: [
    {
      aa: "foo1"
      bb: "bar1"
    },
    {
      aa: "foo2"
      bb: "bar2"
    },
    {
      aa: "foo3"
      bb: "bar3"
    }
    ...
  ],
  b: ...
}

I want to modify some of the fields of the object and left the rest untouched. I'm using the aggregation framework for that, with three stages: unwind of the array, modification of the objects (now at root level in the document) and finally re-grouping by _id to get the array back.

{
  $unwind: "a"
},
{
  $set: {"a.aa": {$concat: ["prefix-", "$a.aa", "-suffix"]}}
},
{
  $group: {
    _id: "$_id",
    a: {$push: "$a"},
    b: {$first: "$b"}
  }
}

So far so good.

The problem is that I don't have only one field in addition to the array. I have lot of them (~10) so the group stage is actually like this:

{
  $group: {
    _id: "$_id",
    a: {$push: "$a"},
    b: {$first: "$b"},
    c: {$first: "$c"},
    d: {$first: "$d"},
    // a lot of fields here
  }
}

which is error prone to write.

Is there some smarter way of doing this without explicitly include all the attributes in the group stage?

Thanks in advance!

Using $unwind followed by {$group: { _id: "$_id", ... is typically a poor approach. Have a look at $filter and $map or $reduce if the operations are more complex. — Wernfried Domscheit
– Wernfried Domscheit, Commented Apr 21, 2023 at 12:39

Wernfried Domscheit · Accepted Answer · 2023-04-21 12:38:52Z

2

Based on your sample data, it could be this one:

db.collection.aggregate([
   {
      $set: {
         a: {
            $map: {
               input: "$a",
               in: {
                  $mergeObjects: [
                     "$$this",
                     { aa: { $concat: ["prefix-", "$$this.aa", "-suffix"] } }
                  ]
               }
            }
         }
      }
   }
])

answered Apr 21, 2023 at 12:38

Wernfried Domscheit

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