Add a RethinkDb example, avoiding pitfalls

Author: kouak

README.md

@@ -452,6 +452,86 @@ Promise.all([ promise1, promise2 ]).then(([ user1, user2]) => {
 });
 ```
 
+#### RethinkDb
+
+RethinkDb offers a batching method called `getAll` but there are a few caveats :
+* Order of results is not guaranteed ([rethinkdb/rethinkdb#5187][rethinkdb_5187])
+* Non-existent keys will not return and empty record
+Assuming a table `example_table` with those records :
+```js
+[
+  {"id": 1, "name": "Document 1"},
+  {"id": 2, "name": "Document 2"}
+]
+```
+A query `r.getAll(1, 2, 3)` could return such an array :
+```js
+[
+  {"id": 2, "name": "Document 2"},
+  {"id": 1, "name": "Document 1"}
+]
+```
+
+In essence, this naive implementation won't work :
+```js
+var r = require('rethinkdb');
+var db = await r.connect();
+
+var batchLoadFn = keys => db.table('example_table').getAll(...keys).then(res => res.toArray());
+var exampleLoader = new DataLoader(batchLoadFn);
+
+await exampleLoader.loadMany([1, 2, 3]); // Throws, values length !== keys length
+
+await exampleLoader.loadMany([1, 2]);
+await exampleLoader.load(1); // {"id": 2, "name": "Document 2"}
+```
+
+A solution is to normalize results returned by `getAll` to match the structure of supplied `keys`.
+
+To achieve this efficiently, we first write an indexing function. This function will return a Map indexing results.
+Parameters :
+* `results` : Array of RethinkDb results
+* `indexField` : String indicating which field was used as index for this batch query
+* `cacheKeyFn` : Optional function used to serialize non-scalar index field values
+```js
+function indexResults(results, indexField, cacheKeyFn = key => key) {
+  var indexedResults = new Map();
+  results.forEach(res => {
+    indexedResults.set(cacheKeyFn(res[indexField]), res);
+  });
+  return indexedResults;
+}
+```
+Then, we can leverage our Map to normalize RethinkDb results with another utility function which will produce a normalizing function.
+```js
+function normalizeRethinkDbResults(keys, indexField, cacheKeyFn = key => key) {
+  return results => {
+    var indexedResults = indexResults(results, indexField, cacheKeyFn);
+    return keys.map(val => indexedResults.get(cacheKeyFn(val)) || new Error(`Key not found : ${val}`));
+  }
+}
+```
+
+Full dataloader implementation :
+```js
+var r = require('rethinkdb');
+var db = await r.connect();
+
+var batchLoadFn = keys => db.table('example_table')
+  .getAll(...keys)
+  .then(res => res.toArray())
+  .then(normalizeRethinkDbResults(keys, 'id'));
+
+var exampleLoader = new DataLoader(batchLoadFn);
+
+await exampleLoader.loadMany([1, 2, 3]); // [{"id": 1, "name": "Document 1"}, {"id": 2, "name": "Document 2"}, Error];
+
+await exampleLoader.load(1); // {"id": 1, "name": "Document 1"}
+```
+
+
+
+
 ## Video Source Code Walkthrough
 
 **DataLoader Source Code Walkthrough (YouTube):**
@@ -468,3 +548,4 @@ Promise.all([ promise1, promise2 ]).then(([ user1, user2]) => {
 [node_redis]: https://github.com/NodeRedis/node_redis
 [nano]: https://github.com/dscape/nano
 [sqlite3]: https://github.com/mapbox/node-sqlite3
+[rethinkdb_5187]: https://github.com/rethinkdb/rethinkdb/issues/5187