dataloader-align-results

Takes your back-end results and returns an Array acceptable to DataLoader.

Installation

npm install dataloader-align-results --save

Documentation

To uphold the constraints of the DataLoader batch function, e.g. new DataLoader(callBatchGetUsers), the batch function must return an Array of values the same length as the Array of keys, and re-order them to ensure each index aligns with the original keys.

The back-end service typically returns results in a different order than we requested, and it typically omits a result for some keys when no value exists for that key.

This utility takes such back-end results and returns an Array acceptable to DataLoader.

dataLoaderAlignResults({ graphqlType, serializeDataLoaderKey, serializeRecordKey, onError });

• grapgqlType (string) - Type of GraphQL result to return for each DataLoader key
  • '[!]!' - An array of non-null objects.
  • '[!]' - An array of non-null objects, or null.
  • '[]!' - Null, else an array of elements each of which is either an object or null.
  • '[]' - An array of elements each of which is either an object or null.
  • '!' - A non-null object.
  • '' - An object or null.
• serializeDataLoaderKey (optional, function(key)) - Function to serialize a key passed from DataLoader.
  • A non-trivial serialization function should more efficiently be passed to DataLoader, e.g. new DataLoader(keys => {/* ... */}, { cacheKeyFn: serializeDataLoaderKey }), than to dataLoaderAlignResults.
  • Defaults to the trivial key => key.toString().
• serializeRecordKey (optional, function(record) or string) - Function to serialize key from the record.
  • The resulting value must be strictly equal to that produced by serializeDataLoaderKey, or passed by DataLoader.
  • If a string is provided, record => record[serializeRecordKey].toString() is used.
• onError (optional, function(msg, detail)) - Handler for terminal errors.
  • Errors are ignored if onError is not provided.
• isStrict (optional, boolean) - It may be more convenient at times to return [] for a missing key rather than null, as the null may require special processing in the calling code.
  • true - Return null.
  • else - Return [].

dataLoaderAlignResults returns a function(keys, resultsArray) which should be passed to DataLoader as its first parameter.

• keys - The keys passed from the DataLoader.
• resultsArray - Result from back-end service after processing the keys.
  • Results are likely in a different order than that of keys.
  • A result may be omitted for some keys.

The batch loader function calls onError if it finds results incompatible with graphqlType. However it cannot check for a null final result for '[]!' or '[!]!' because DataLoader itself builds the final Array from elements returned by the batch loader plus elements from the cache.

Complete Example

const DataLoader = require('dataloader');
const dataloaderAlignResults = require('dataloader-align-results');
 
const userLoader = new DataLoader(keys => myBatchGetUsers(keys));
const userAlignResults = dataloaderAlignResults({
  graphqlType: '!', onError: (msg, detail) => { throw new Error(`${msg}\n${detail}`); } });
 
// These logical reads will be resolved with one call to the back-end service.
Promise.all([
  userLoader.load(3),
  userLoader.load(2),
  userLoader.load(4),
  userLoader.load(1),
])
  .then(resolvedResults => { /* ... */ });

 
function myBatchGetUsers(keys) {
  return callBatchGetUsers(keys) // Call back-end service.
    .then(resultsArray => userAlignResults(resultsArray, keys));
}

License

Copyright (c) 2017

Licensed under the MIT license.