[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [RefCount](https://evolu.dev/docs/api-reference/common/RefCount) › CreateRefCountByKeyOptions

Defined in: [packages/common/src/RefCount.ts:84](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/RefCount.ts#L84)

Options for [createRefCountByKey](https://evolu.dev/docs/api-reference/common/RefCount/functions/createRefCountByKey).

## Extends

- [`LookupOption`](https://evolu.dev/docs/api-reference/common/Lookup/interfaces/LookupOption)\<`TKey`, `L`\>

## Properties

<a id="lookup"></a>

### lookup?

```ts
readonly optional lookup?: Lookup<TKey, L>;
```

Defined in: [packages/common/src/Lookup.ts:44](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Lookup.ts#L44)

Derives the lookup key used for logical equality.

#### Inherited from

[`LookupOption`](https://evolu.dev/docs/api-reference/common/Lookup/interfaces/LookupOption).[`lookup`](https://evolu.dev/docs/api-reference/common/Lookup/interfaces/LookupOption#lookup)