New filter for [not] null

[wolfgang42]

I have a need to filter a field for items which are not null. However, there is currently no filter syntax for either null or not null items. I can submit a pull request to add this feature, but I'm not sure what a suitable syntax is. I have thought of either filter[field]=*exists or filter[field]=!*null (adding the ! syntax for all filters, not just *null), but would like to seek feedback on this before implementing it.
Which syntax is preferred (or should we use something else altogether)?

[BryanHunt]

I just encountered this and it's now blocking me. I need to find objects where a relationship does not exist.

[wolfgang42]

I wound up building my own custom filter handling, overriding jsonapi-server's filtering entirely (for various reasons not entirely to do with limitations of this library), so I won't be contributing a PR for this as I originally stated. However I still think it is a useful feature to have.

Here's the syntax I settled on:

filter[field]=value
filter[field][exists]=true
filter[field][in]=["value1","value2"] // JSON-encoded array

This has the advantage that it's A) easily extendable, and B) doesn't require any special escaping.

Also, I haven't needed it yet but you could easily add a [not] filter:

filter[field][not]=value
filter[field][not][in]=[1,2,3]