Array-type resolvers: an intuitive solution to the n+1 problem

[Ustice]

There have been a lot of solutions to the n+1 problem in GraphQL. Patterns like the DataLoader gather up the requests and allow the developer to make a single request for the group. These come with their own complications, and learning curve.

If we include the main idea of these solutions at the GraphQL level, we can get similar capabilities all with reduced complexity. To accomplish this we need to decouple the list type from the non-list type, which allows developers to make a single external request for a list of results.

List-field resolvers would work just a little differently from the typical resolver. When a request is being parsed, if the field type is a list type, then we should check the resolvers for a list of objects of that type. If such a resolver is found then we first resolve list, and call the list field resolver rather than mapping over the individual resolvers for every item in that field. The resolver would then call the resolver with a parent object that is a list, and one which must resolve to an array of the same length, where each element corresponds to the field for value for the parent of the same index. Returning an array of a different length would result in an error being thrown, as it would be impossible to field values back onto the parent object.

This is a lot more difficult to describe than to show an example. So let's consider this schema:

extend type Query {
  things: [Thing!]!
  thing(id: String!): Thing
}

type Thing {
  id: String!
  property: String!
  stuff: [Stuff!]!
}

type Stuff {
  id: String!
  description: String!
}

I'm going to use prisma for the example, and we can assume that the structure in the database matches the GraphQL objects. Given that, here

const resolvers = {
  Query: {
    thing: (__, { id }) => prisma.thing.findOne({ where: { id } })
    things: () => prisma.thing.findMany() 
  },
  Thing: {
    stuff: ({ id }) => prisma.stuff.findMany({ where: { thingId: id } }),
    
    // This is an approximation of the default field resolver,
    // used for comparison to the array-field defined below
    property: (thing) => thing.property
  },
  
  // When a field is of type `[Thing]`, these are the resolvers for the subfields. 
  "[Thing]": {
    stuff: async (things) => {
      const parentIds = things.map((n) => n.id)
      const allStuff = await prisma.stuff.findMany({ where: { thingId: { in: parentsIds } } })

      // Notice how we are not just returning an array, but instead an array of arrays,
      // where for _each_ parent element, we return the value of the corresponding field.
      // The easiest way to accomplish this is to simply map over the parent list (`things`)
      return things.map((thing) => allStuff.filter(({ thingId }) => thingId === thing.id ))
      
      // This would be an error as `allStuff` is of type `Stuff[]`, but the resolver expects a return
      //  value of type `Stuff[][]`
      return allStuff
    },
    
    // This is an approximation of what would become the default resolver for `[Things].property`
    property: async (things, args, context, info) => 
      things.map(
        // Don't actually do this, but this illustrates how list-fields are resolved by default.
        (thing) => resolvers.Thing.property(thing, args, context, info)
      )
  },
}

And finally the following query:

  Query ThingsWithStuff {
    things {
      id
      property
      stuff {
        id
        description
      }
    }
  }

As far as I am aware all GraphQL servers in the Javascript ecosystem, to resolve this query would call resolvers.Query.things, and then for each result would call, resolvers.Thing.stuff for as many things that are returned from the first query, ie the infamous n+1 problem.

My hypothetical GraphQL server would again start by calling resolvers.Query.things, but instead this time it calls resolvers["[Thing]"].stuff, resulting in just two database queries.

If a more complete example with data is necessary I can write that up later.

[yaacovCR]

One complication is that a field can return a list of interfaces, where items may include different types of objects at runtime that fulfill the abstract type, with possibly different selection sets.

if you have a proposal for modifying the execution flow, of course, you can always submit a PR for a review. This repository tries to match the specification as much as possible, but an optimizations that do not obfuscate too much can always be considered.

It might make sense for the PR within this repository to simply allow for a more flexible architecture where the actual change of execution flow is a separate user space package.