Candidate Search sort option 500's

[cnlucas]

What we’re after

We have been having a user hit our candidate search endpoint and try to search by receipts. This passes validation, but is not a feasible option so error's out.

The call:
https://api.open.fec.gov/v1/candidates/search/?page=1&per_page=20&sort=ways&sort_hide_null=false&sort_null_only=false&sort_nulls_last=false&api_key=DEMO_KEY

In candidates.py line 45
aliases = {'receipts': models.CandidateSearch.receipts}

Action item(s)

(These are the smaller tasks that should happen in order to complete this work)

• Research why this bug is occurring and fix it
• Decide to either fix the receipts sorting or remove it from the validation

Completion criteria

(What does the end state look like - as long as this task(s) is done, this work is complete)

• Candidate search either works for receipts sorting or will 422

References/resources/technical considerations

(Is there sample code or a screenshot you can include to highlight a particular issue? Here is where you reinforce why this work is important)

[fec-jli]

1. /candidates/ endpoint throws 500 error, (when sort=receipts , without q), sample url:
   http://127.0.0.1:5000/v1/candidates/?page=1&per_page=20&sort=receipts&sort_hide_null=false&sort_null_only=false&sort_nulls_last=false

2. /candidates/search/endpoint throws 500 error, (when sort=receipts , without q), sample url:
   http://127.0.0.1:5000/v1/candidates/search/?page=1&per_page=20&sort=receipts&sort_hide_null=false&sort_null_only=false&sort_nulls_last=false&api_key=DEMO_KEY

view class: candidates.py line 37: CandidateList(ApiResource)
model class: models.Candidate and models.CandidateSearch

https://github.com/fecgov/openFEC/blob/develop/webservices/resources/candidates.py#L72-L74

The root cause:
before running select count(*) query, we should validate argument first,

        if {'receipts', '-receipts'}.intersection(
            kwargs.get('sort', [])
        ) and 'q' not in kwargs:
            raise exceptions.ApiError(
                'Cannot sort on receipts when parameter "q" is not set',
                status_code=422,
            )