loaders.md

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:

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:

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.