Adding maxBatchSize to options

README.md

@@ -160,6 +160,9 @@ Create a new `DataLoader` given a batch loading function and options.
   - *batch*: Default `true`. Set to `false` to disable batching, instead
     immediately invoking `batchLoadFn` with a single load key.
 
+  - *maxBatchSize*: Default infinite. Limits the number of items that get
+    passed in to the `batchLoadFn`.
+
   - *cache*: Default `true`. Set to `false` to disable caching, instead
     creating a new Promise and new key in the `batchLoadFn` for every load.
 

src/__tests__/dataloader-test.js

@@ -62,6 +62,22 @@ describe('Primary API', () => {
     expect(loadCalls).to.deep.equal([ [ 1, 2 ] ]);
   });
 
+  it('batches multiple requests with max batch sizes', async () => {
+    var [ identityLoader, loadCalls ] = idLoader({ maxBatchSize: 2 });
+
+    var promise1 = identityLoader.load(1);
+    var promise2 = identityLoader.load(2);
+    var promise3 = identityLoader.load(3);
+
+    var [ value1, value2, value3 ] =
+      await Promise.all([ promise1, promise2, promise3 ]);
+    expect(value1).to.equal(1);
+    expect(value2).to.equal(2);
+    expect(value3).to.equal(3);
+
+    expect(loadCalls).to.deep.equal([ [ 1, 2 ], [ 3 ] ]);
+  });
+
   it('coalesces identical requests', async () => {
     var [ identityLoader, loadCalls ] = idLoader();
 

src/index.js

@@ -19,6 +19,7 @@ type Options<K, V> = {
   cache?: boolean;
   cacheKeyFn?: (key: any) => any;
   cacheMap?: CacheMap<K, Promise<V>>;
+  maxBatchSize?: number;
 };
 
 // If a custom cache is provided, it must be of this type (a subset of ES6 Map).
@@ -220,6 +221,14 @@ function dispatchQueue<K, V>(loader: DataLoader<K, V>) {
   var queue = loader._queue;
   loader._queue = [];
 
+  var maxBatchSize = loader._options && loader._options.maxBatchSize ||
+    queue.length;
+  for (var i = 0; i < queue.length / maxBatchSize; i++) {
+    loadKeys(loader, queue.slice(i * maxBatchSize, (i + 1) * maxBatchSize));
+  }
+}
+
+function loadKeys<K, V>(loader: DataLoader<K, V>, queue: LoaderQueue<K, V>) {
   // Collect all keys to be loaded in this dispatch
   var keys = queue.map(({ key }) => key);