Trigger custom filter when no value is submitted

[linusdm]

Hey,

I've got a derived Flop schema that specifies a custom field for filtering. The challenge I'm presented with, is that I want to apply a default where-constraint when no filter is chosen at all (e.g. the respective query parameter in the url is not set). There is no one-to-one relation between the chosen filter value in the UI and the actual where-constraints (hence the need for custom logic, and why a regular field won't work). In my case, not choosing a filter value in the UI should also apply a where-constraint. I don't want to apply the default filter if a certain value is chosen for the filter (this is just to say that the default filter is not always present).

I'm not sure how to go about this. Right now I manipulate the query myself, after inspecting the Flop.filters field (I run Flop.validate/2 explicitly, and then later Flop.run/3 with the modified query). In addition, I added a custom field with a no-op filter (just returning the unmodified the input query). This basically is a hack to shoehorn my custom logic into the Flop way of passing in filter values.

Am I missing something obvious here? I think Flop chooses somewhere deliberately that a custom field filter is not triggered, if there is no value detected. This probably works 99% of the times.

I welcome any suggestion to improve my existing solution.