Implement a new prime(key, value) API, to allow for directly priming

the cache with a given key and value.

Author: Ryan Spradlin

README.md

@@ -196,6 +196,11 @@ Clears the entire cache. To be used when some event results in unknown
 invalidations across this particular `DataLoader`. Returns itself for
 method chaining.
 
+##### `prime(key, value)`
+
+Primes the cache with the provided key and value. If the key already exists, the
+value will be updated. Returns itself for method chaining.
+
 
 ## Using with GraphQL
 

src/__tests__/dataloader-test.js

@@ -165,6 +165,22 @@ describe('Primary API', () => {
     expect(loadCalls).to.deep.equal([ [ 'A', 'B' ], [ 'A', 'B' ] ]);
   });
 
+  it('allows priming the cache', async () => {
+    var [ identityLoader, loadCalls ] = idLoader();
+
+    identityLoader.prime('A', 'A');
+
+    var [ a, b ] = await Promise.all([
+      identityLoader.load('A'),
+      identityLoader.load('B')
+    ]);
+
+    expect(a).to.equal('A');
+    expect(b).to.equal('B');
+
+    expect(loadCalls).to.deep.equal([ [ 'B' ] ]);
+  });
+
 });
 
 describe('Represents Errors', () => {
@@ -249,6 +265,23 @@ describe('Represents Errors', () => {
     expect(loadCalls).to.deep.equal([ [ 1 ] ]);
   });
 
+  it('Handles priming the cache with an error', async () => {
+    var [ identityLoader, loadCalls ] = idLoader();
+
+    identityLoader.prime(1, new Error('Error: 1'));
+
+    var caughtErrorA;
+    try {
+      await identityLoader.load(1);
+    } catch (error) {
+      caughtErrorA = error;
+    }
+    expect(caughtErrorA).to.be.instanceof(Error);
+    expect((caughtErrorA: any).message).to.equal('Error: 1');
+
+    expect(loadCalls).to.deep.equal([]);
+  });
+
   it('Can clear values from cache after errors', async () => {
     var loadCalls = [];
     var errorLoader = new DataLoader(keys => {
@@ -485,6 +518,22 @@ describe('Accepts options', () => {
       expect(identityLoadCalls[0][0]).to.equal(keyA);
     });
 
+    it('Allows priming the cache with an object key', async () => {
+      var [ identityLoader, loadCalls ] = idLoader({ cacheKeyFn: cacheKey });
+
+      var key1 = { id: 123 };
+      var key2 = { id: 123 };
+
+      identityLoader.prime(key1, key1);
+
+      var value1 = await identityLoader.load(key1);
+      var value2 = await identityLoader.load(key2);
+
+      expect(loadCalls).to.deep.equal([]);
+      expect(value1).to.equal(key1);
+      expect(value2).to.equal(key1);
+    });
+
   });
 
   describe('Accepts custom cacheMap instance', () => {

src/index.js

@@ -10,14 +10,14 @@
 
 // A Function, which when given an Array of keys, returns a Promise of an Array
 // of values or Errors.
-type BatchLoadFn<K, V> = (keys: Array<K>) => Promise<Array<V | Error>>
+type BatchLoadFn<K, V> = (keys: Array<K>) => Promise<Array<V | Error>>;
 
 type CacheMap<K, V> = {
   get(key: K): V | void;
   set(key: K, value: V): any;
   delete(key: K): any;
   clear(): any;
-}
+};
 
 // Optionally turn off batching or caching or provide a cache key function or a
 // custom cache instance.
@@ -26,7 +26,7 @@ type Options<K, V> = {
   cache?: boolean,
   cacheMap?: CacheMap<K, Promise<V>>,
   cacheKeyFn?: (key: any) => any
-}
+};
 
 /**
  * A `DataLoader` creates a public API for loading data from a particular
@@ -157,6 +157,24 @@ export default class DataLoader<K, V> {
     this._promiseCache.clear();
     return this;
   }
+
+  /**
+   * Adds/updates the provied key and value in the cache. Returns itself for
+   * method chaining.
+   */
+  prime(key: K, value: V): DataLoader<K, V> {
+    var cacheKeyFn = this._options && this._options.cacheKeyFn;
+    var cacheKey = cacheKeyFn ? cacheKeyFn(key) : key;
+
+    // Cache a rejected promise if the value is an Error, in order to match the
+    // behavior of load(key).
+    var promise = value instanceof Error ?
+        Promise.reject(value) :
+        Promise.resolve(value);
+
+    this._promiseCache.set(cacheKey, promise);
+    return this;
+  }
 }
 
 // Private: Enqueue a Job to be executed after all "PromiseJobs" Jobs.