How do I pass custom options into the batch function DataLoader uses to resolve resources?

I have a circumstance where we want to return a modified result based on a graphQL variable. It appears that the loaders only take in an id argument, leaving no way to pass through other options for the server to consume. Ideally, I’d like to do something like this:

const postLoader = new DataLoader((keys, options) => {
  ...return promise with posts
});

...
const ids = [1,2,3];
postLoader.loadMany(ids, { withCommentType: 'pending' });

The way I have solved it now is to create a separate loader for each option, but this solution seems far less scalable:

const postLoader = new DataLoader((keys) => {
  ...return promise with posts
});
const postWithPendingCommentsLoader = new DataLoader((keys) => {
  ...return promise with posts including comments that are pending review
});

...
const ids = [1,2,3];
postWithPendingCommentsLoader.loadMany(ids);

Are there any plans to allow an options passthrough like this in the future? Or am I missing an alternate solution?

[leebyron]

Why isn’t there an option to pass in a contextual object?

Typically DataLoader instances are creates as part of a request context, in which case other elements of a context can be referenced directly without the need to pass them in. Passing them in by call would require fairly complex logic to partition batches based on equivalent contexts. Instead it’s preferred to just create a new instance per context.

To the original question, DataLoader expects a strict key -> value relationship and does not support additional arguments. If additional arguments are needed, they can be considered part of the key (example, { id: 1, withCommentType: 'pending' }) so they can be considered different keys (and thus cached differently) from keys that might represent a similar object with different arguments.

Alternatively (as well discussed above) if there are a small number of potential values for an argument, multiple DataLoaders can be created, one per potential value. However this may depend on your application domain.

[RAMPKORV]

We could have a set of batch functions like this.

const batchFunctions = {
  getBooks: (con) => async (ids) => {
    let [ rows ] = await con.query('SELECT * FROM books WHERE id IN ? ORDER BY FIELD(id, ?)', ids, ids);
    return rows;
  }
}

To turn it into an object of data loaders we do

let loaders = createLoadersFromBatchFunctions(batchFunctions, {})

And then we can load books in this manner;

let bookFromOldDb = await loaders.getBooks(legacyDbConnection).load(6);
let bookFromNewDb = await loaders.getBooks(newDbConnection).load(4);

How it works? When the dataloader is created, we create a batch function with ids => fnRef(ids), but fnRef will be created at runtime when we run something like loaders.getBooks(newDbConnection)