[REQUEST] Add batch grouping

[GNevsky]

What problem are you trying to solve?

Handling different parameters in field resolvers with dataloader is tricky.

Let’s have this GraphQL schema:

type Order {
  orderId: Int
  status: String
}

type User {
  userId: Int
  name: String
  orders(status: String): [Order!]
}

type Query {
  GetUsers(search: String): [User!]
}

schema {
  query: Query
}

Then we have query:

query {
    GetUsers(search: "john") {
        userId
        name
        ordersNew: orders(status: "New") {
            orderId
            status
        }
        ordersCompleted: orders(status: "Completed") {
            orderId
            status
        }
    }
}

Here orders resolver with dataloader:

User: {
    orders: async ({ userId }, { status }) => {
      return await ordersLoader.load({userId: userId, status: status});
    }
  },

Here ordersLoader with batchLoadFn:

const ordersLoader = new DataLoader(
  async (params: { userId: number, status?: string }[]): Promise<Order[][]> => {
...
  }
);

In this batch loader function we need to get orders from a database. If we have all orders stored in one table we can have single query, like:

SELECT * from Orders WHERE userId IN (:userIds) and status IN (:statuses)

Then loop through params and return the result. No real issues, so far.

Now let’s think orders in different statuses can’t be queried with a single query. For example, “New” orders are in “Online” db and “Competed” are moved in some “Archive” db. So, we need to have 2 queries. Then we need to build results properly based on input params. It is doable but even with this simple example not so easy. In real life we may have multiple parameters and we need to handle all permutations of them.

Describe the solution you'd like

I suggest adding a new option to DataLoader constructor. Very similar to existing cacheKeyFn – batchGroupFn?: (key: K) => C;. Based on result of this function batchLoadFn function needs to be called as many times as many different results gives batchGroupFn. I.e. I want to group batches by input keys.

For my example it would be:

batchGroupFn: (key: { userId: number, status?: string }) => (key.status??"")

So for each status it will be separate call of batchLoadFn. It will simplify this function a lot.

Please let me know if this solution is feasible. Or maybe there is alternative/better solution?