Dataloader.KV: load new items when results are non-empty

[alex-knowles]

Fixes issue #74.

Map.get/2 will either return some value() or nil.

dataloader/lib/dataloader/kv.ex

Lines 85 to 88 in 56f7cb4

	case Map.get(batch, id) do
	{:error, reason} -> {:error, reason}
	item -> {:ok, item}
	end

The above case statement does not specifically handle nil, because it (incorrectly) expects to find either an :error 2-tuple or a cached result.

However, when fetch/3 is called by load/3, it is possible that:

1. results is a non-empty map, and
2. there is no results entry matching id

In that case, Map.get/2 would return nil, but it was being pattern-matched as a positive result instead of an error, which caused the incorrect path to be followed in:

dataloader/lib/dataloader/kv.ex

Lines 72 to 80 in 56f7cb4

	case fetch(source, batch_key, id) do
	{:error, _message} ->
	update_in(source.batches, fn batches ->
	Map.update(batches, batch_key, MapSet.new([id]), &MapSet.put(&1, id))
	end)
	_ ->
	source
	end

This prevented new data from being loaded into a Dataloader.KV source that contained cached results.

The fix proposed here is to use Map.get/3 with a third argument of :error, which can be explicitly matched in the case statement:

        case Map.get(batch, id, :error) do
          {:error, reason} -> {:error, reason}
          :error -> {:error, "Unable to find id #{inspect(id)}"}
          item -> {:ok, item}
        end

[benwilson512]

Thank you!