https://blog.boopathi.in/graphql-optimization-dataloaders/

graphql-optimization-dataloaders.js

const productLoader = new DataLoader(getBackendProducts);

const resolvers = {
  Query: {
    async product(_, { id }, __, info) {
      const fields = getFields(info);
      const backendFields = getBackendFields(fields, dependencyMap);
      const backendResponse = await productLoader.load({ id, fields: backendFields });
      const schemaResponse = getSchemaResponse(backendResponse, fields, transformerMap);
      return schemaResponse;
    }
  }
};

const dependencyMap = {
  name: ["title"],
  price: ["price.currency", "price.amount"],
  stock: ["stock_availability"],
};

const transformerMap = {
  name: resp => resp.title,
  price: resp => `${resp.currency} ${resp.amount}`,
  stock: resp => resp.stock_availability,
};

function getFields(info) {
  return info
    .fieldNodes[0] // TODO: handle all field nodes in other fragments
    .selectionSet
    .selections
    .map(
        selection => // TODO: handle fragments
            selection.name.value
    );
}

function getBackendFields(schemaFields, dependencyMap) {
  // Set helps in deduping
  const backendFields = new Set(
    schemaFields
      .map(field => dependencyMap[field])
      .reduce((acc, field) => [...acc, ...field], [])
  );
  return backendFields;
}

async function getBackendProducts(inputs) {
  const {ids, mergedFields} = mergeInputs(inputs);
  const products = await fetch(`/products?ids=${ids.join(",")}&fields=${mergedFields}`);
  return products;
}

function mergeInputs(inputs) {
  const ids = [];
  const fields = new Set();
  for (const input of inputs) {
    ids.push(input.ids);
    for (const field of input.fields) {
      fields.add(field);
    }
  }

  return {
    ids,
    mergedFields: [...fields].join(",")
  };
}

function getSchemaResponse(
  backendResponse,
  transformerMap,
  schemaFields
) {
  const schemaResponse = {};
  for (const field of schemaFields) {
    schemaResponse[field] = transformerMap[field](backendResponse);
  }
  return schemaResponse;
}

Hey Boopathi, Why not use field resolving instead of a mapper (getSchemaResponse) that will map each field with the relevant value from the backend?
The power of Graphql relies in its ability to resolve each field in a different way (from a different data source)

@eelayoubi Completely true. If you have a single data source for the root field, this will just work. The problem we had was a bit more complicated - We have multiple data sources for a field like product - the data source is picked based on the id. Each data source has a different dependency map and a transformer map. A single resolver for a field would work if there is only one of such things.

I'm looking at further ways to ease this process of writing code.