[RFC] Performance improvements: Answer visibility denormalisation

[winged]

When validating documents and answers, often we need to verify that all answers are valid (ever answer where is_required evaluates to True must be filled in, for example)

Now in complex structures, this becomes rather complex, as things like dynamic options, data sources etc come into play.

Proposal

We need to cache locally the resulting value of is_hidden and is_required in answers, as well as is_hidden in options.

To do this, I propose a similar structure to what we already have in the calculated questions: We start with a list of question slugs mentioned in the JEXL expressions. These questions then get a forward reference to our question with the expression.

Each time an answer with such a question is saved, we look for "dependent" answers and invalidate their is_hidden and is_required state, as well as their options' is_hidden if it's a [multiple]choice question.

The document/answer validator then checks the cached values instead of evaluating the JEXL every time:

• if it's None, recalculate and save the result to database
• if it's True or False, no recalculation required, just use the value as-is.