When should you use nested instead of object?

Question

IT Interview Practice · Accepted Answer

Choose `nested` when several conditions must apply to the same element in an array. Otherwise values from different elements can accidentally satisfy the query together.

## A product with two variants

```json
{"variants":[{"color":"red","size":"S"},{"color":"blue","size":"L"}]}
```

There is no red size L variant. But ordinary object mapping flattens these fields into multiple colors and multiple sizes, losing which size belongs to which color. A query for red and L can therefore match the product incorrectly.

## Require both conditions on one variant

Map `variants` as `nested` and `color`/`size` as keyword. Then use:

```json
{
  "query": {
    "nested": {
      "path": "variants",
      "query": {
        "bool": {
          "filter": [
            {"term":{"variants.color":"red"}},
            {"term":{"variants.size":"L"}}
          ]
        }
      }
    }
  }
}
```

Both filters now run inside one nested element, so the example product does not match. Change L to S and it can match the red variant.

## Pay for nested only when needed

Nested elements create additional internal documents and more expensive queries. Ordinary object is simpler when preserving relationships inside arrays is unnecessary. Test both valid and cross-element combinations before deciding the mapping works.

[Official documentation](https://www.elastic.co/docs/reference/elasticsearch/mapping-reference/nested).

A product with two variants

Require both conditions on one variant

Pay for nested only when needed

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