RethinkDB.md

RethinkDb

RethinkDb offers a batching method called getAll but there are a few caveats :

• Order of results is not guaranteed (rethinkdb/rethinkdb#5187)
• Non-existent keys will not return and empty record Assuming a table example_table with those records :

[
  {"id": 1, "name": "Document 1"},
  {"id": 2, "name": "Document 2"}
]

A query r.getAll(1, 2, 3) could return such an array :

[
  {"id": 2, "name": "Document 2"},
  {"id": 1, "name": "Document 1"}
]

In essence, this naive implementation won't work :

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

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.

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 :

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"}