# mercurius

## Loaders

A loader is an utility to avoid the 1 + N query problem of GraphQL.
Each defined loader will register a resolver that coalesces each of the
request and combines them into a single, bulk query. Moreover, it can
also cache the results, so that other parts of the GraphQL do not have
to fetch the same data.

Each loader function has the signature `loader(queries, context)`.
`queries` is an array of objects defined as `{ obj, params }` where
`obj` is the current object and `params` are the GraphQL params (those
are the first two parameters of a normal resolver). The `context` is the
GraphQL context, and it includes a `reply` object.

Example:

```js
const loaders = {
  Dog: {
    async owner(queries, { reply }) {
      return queries.map(({ obj }) => owners[obj.name])
    }
  }
}

app.register(mercurius, {
  schema,
  resolvers,
  loaders
})
```

It is also possible disable caching with:

```js
const loaders = {
  Dog: {
    owner: {
      async loader(queries, { reply }) {
        return queries.map(({ obj }) => owners[obj.name])
      },
      opts: {
        cache: false
      }
    }
  }
}

app.register(mercurius, {
  schema,
  resolvers,
  loaders
})
```

Disabling caching has the advantage to avoid the serialization at
the cost of more objects to fetch in the resolvers.

Internally, it uses
[single-user-cache](http://npm.im/single-user-cache).