ScoreFunction is not documented - Unknown query [function_score]

[andyhmltn]

Hello,

I'm trying to implement a function score using elasticsearch-dsl-py. So I'm trying to use ScoreFunction like so:

score_function = ScoreFunction(weight=5, filter=Range(linked_user__last_login={'gt': 'now-1y'}))

However if I call to_dict on that method I can see the dictionary returned has None as a key which seems wrong:

{None: {},
 'filter': {'range': {'linked_user.last_login': {'gt': 'now-1y'}}},
 'weight': 5}

Then if I try to use that with my search like so (after a hint from here: #608 (comment)):

search.query = FunctionScore(query=search.query, functions=[score_function])

and call search.execute() I get:

RequestError(400, 'parsing_exception', 'unknown query [function_score] did you mean any of [random_score, script_score]?')

What's the cause for this? The only documentation I can find on this is in issue #608 and using the example results in the following error.

Thank you!