Feature/sparql bugs

[JeltevanBoheemen]

Fixes for various SPARQL bugs (#394 #423 #469).

Clunky fix for wrong arity SELECT queries with Accept: text/turtle. See #394 (comment), let me know if you can't live with this hackiness 😄

@BeritJanssen Turns out the nlp-ontology.graph() for local testing caused most tests to fail, because it keeps resetting to the example file (was a bit of detective work to find this). Rather than try to fix that, I just moved the tests to a different endpoint.

[JeltevanBoheemen]

This is fairly hacky. Couldn't figure out how to do this more elegantly, as the error occurs very late: when the response is already formed, only the content needs to be rendered.

[jgonggrijp]

You mean it is too late to catch the exception and re-raise as a ValidationError? In that case, perhaps you can emulate that mechanism with something like the following:

# top of module
from rest_framework.exceptions import ValidationError
from ..rdf.views import custom_exception_handler

# here
         try:
             results_graph = graph_from_triples(query_results)
         except Exception as e:
             error = ValidationError(str(e))
             response = custom_exception_handler(error, renderer_context)
             results_graph = response.data
             renderer_context['response'] = response
             return super().render(results_graph, media_type, renderer_context)

[jgonggrijp]

Obviously I can't really object against a PR that fixes three bugs, but yes, the part you commented on does look a bit hacky.

[jgonggrijp]

You mean it is too late to catch the exception and re-raise as a ValidationError? In that case, perhaps you can emulate that mechanism with something like the following:

# top of module
from rest_framework.exceptions import ValidationError
from ..rdf.views import custom_exception_handler

# here
         try:
             results_graph = graph_from_triples(query_results)
         except Exception as e:
             error = ValidationError(str(e))
             response = custom_exception_handler(error, renderer_context)
             results_graph = response.data
             renderer_context['response'] = response
             return super().render(results_graph, media_type, renderer_context)

[jgonggrijp]

This looks somewhat familiar!

[JeltevanBoheemen]

@jgonggrijp Thanks for the suggestion. That probably would have worked.
However, I chose a different approach. The view will have to re-negotiate to account for query type, this is 'hacky' by nature. Instead of doing this in finalize_response, it is now done in execute_query. Note that I chose finalize_response because it re-negotiates if there is no Accept provided in the request, and enforces the first available. By setting the accepted renderer in an earlier stage, this is skipped. This` fits much better in the rest_framework flow, and avoids hacky response overwrites.

[jgonggrijp]

Always nice when so much code can go!