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, info } where obj is the current object, params are the GraphQL params (those are the first two parameters of a normal resolver) and info contains additional information about the query and execution. info object is only available in the loader if the cache is set to false. The context is the GraphQL context, and it includes a reply object.

Example:

const loaders = {
  Dog: {
    async owner (queries, { reply }) {
      return queries.map(({ obj, params }) => 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, params, info }) => { 
          // info is available only if the loader is not cached
          owners[obj.name]
        })
      },
      opts: {
        cache: false
      }
    }
  }
}

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

Alternatively, globally disabling caching also disable the Loader cache:

const loaders = {
  Dog: {
    async owner (queries, { reply }) {
      return queries.map(({ obj, params, info }) => { 
        // info is available only if the loader is not cached
        owners[obj.name]
      })
    }
  }
}

app.register(mercurius, {
  schema,
  resolvers,
  loaders,
  cache: false
})

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.