Getting array of values from array of objects containing a certain key-value pair

Question

Let's say I have an Array of Objects chars: [{'name':'tom','age':2},{'name':'jerry','age':3}]

What's the aggregation to get chars: ['tom','jerry'] or chars: 'tom,jerry' ?

I know I can get chars: [{'name':'tom'},{'name':'jerry'}] by $project on chars.name, but I want a more compact way to see what I have in the values since the keys are uninteresting.

nimrod serok · Accepted Answer · 2022-05-23 18:47:08Z

1

You can use $reduce with $concatArrays for this:

db.collection.aggregate([
  {
    $project: {
      chars: {
        $reduce: {
          input: "$chars",
          initialValue: [],
          in: {
            $concatArrays: [
              "$$value",
              [
                "$$this.name"
              ]
            ]
          }
        }
      },
      _id: 0
    }
  }
])

Playground example

The $reduce is iterating array elements and create one result from them.

answered May 23, 2022 at 18:47

nimrod serok

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