JSON filter expressions

[tschaub]

Here is a proposed syntax for the JSON encoded filter expressions. Please see the readme for a starting point.

I've included valid and invalid fixtures for tests against the schema (run the tests with python test.py). The doc.py script extracts title, description, and examples for expressions in the schema. I've pasted this output in the readme linked above.

As mentioned in the sprint presentations, I was shooting for feature parity with CQL (which I hope I have demonstrated is straightforward - if there are missing operators, they can be added). I was also hoping to encourage some level of interoperability with expressions in the Mapbox Style Spec. If OGC is considering a general purpose "filter" extension, and will later potentially be reconsidering some sort of "style" extension, I think it makes sense to aim for interoperability with the Mapbox Style Spec where possible. The JSON filter expressions I've proposed here would fit nicely with a JSON encoding for styles that is interoperable with Mapbox.

The readme links to these as well, but here are few brave implementors:

• National Land Survey of Finland OGC API – Features implementation at https://beta-paikkatieto.maanmittauslaitos.fi/maastotiedot/features/v1/ with experimental support for filter-lang=json-filter-expr. E.g. https://beta-paikkatieto.maanmittauslaitos.fi/maastotiedot/features/v1/collections/tieviiva/items?limit=100&filter-lang=json-filter-expr&filter=["==",["get","kohdeluokka"],12316]

• Experimental support in GDAL (using filter-lang=json-filter-expr) - OAPIF: implement https://github.com/tschaub/ogcapi-features/blob/json-array-expression/extensions/cql/jfe/readme.md OSGeo/gdal#1999

• Generic Golang parser for JSON (Filter) Expressions - https://github.com/tschaub/jiffy

• Support for JSON Filter Expression parsing in pygeofilter - https://github.com/geopython/pygeofilter

[pvretano]

Tim, why does the pull request say that the changed file(s) are under extensions/filter/languages/jfe ...? All this stuff should be under the extensions/cql directory. Having all this preliminary work, all related to the same topic, spread across multiple directories will -- in my opinion -- really confuse the work of of the SWG. Feel free to reorgnize the files under extensions/cql though (expect the standard) directory.

[tschaub]

Will do @pvretano

I thought the extension was going to be called "filter" and that one of the supported languages would be CQL. I'll move it back under extensions/cql if that is less confusing though.

[pvretano]

Thanks @tschaub . My feeling is that CQL -- although started in OAPIF -- will end up in OGC API - Common at which point that SWG will organize as they see fit. For now it probably best to keep everything CQL related in one place.