DataLoader.load(None) explicitly disallowed.

[bryanhelmig]

This refers to this section of code:

promise/promise/dataloader.py

Lines 52 to 60 in b055d1e

	def load(self, key=None):
	'''
	Loads a key, returning a `Promise` for the value represented by that key.
	'''
	if key is None:
	raise TypeError((
	'The loader.load() function must be called with a value,' +
	'but got: {}.'
	).format(key))

Which is rather limiting, consider an ORM with nullable FK:

class SomeObject(models.Model):
    friend = models.ForeignKey(SomeObject, null=True, blank=True)
    friends = models.ManyToManyField(SomeObject, blank=True)

def some_object_resolver(parent, info, **kwargs):
    # raises error when load is None! sad :(
    return dataloader.load(parent.friend_id)

def some_object_resolver(parent, info, **kwargs):
    # empty iterable is fine? yay? :)
    return dataloader.load_many([
         friend.id for friend in parent.friends.all()
    ])

For the most reusable DataLoaders it would be nice to allow None to pass through such that batch_load_fn() handles empty values without an issue.

I suppose the alternative is just to return None directly from the resolver if the friend_id is None, but for reusableness' sake it would be awesome to not add that conditional.

[bryanhelmig]

@syrusakbary for your consideration!