null not equal empty string (back compatibility issue)

[springmeyer]

In Mapnik 2.2 you can now do [attr] != null to filter out true nulls after the fixes as part of #1642. But in older Mapnik you could not do [attr] != null and instead had to match null by doing [attr] != ''. Now [attr] != '' only rejects empty strings and actually matches nulls since null != ''. Whether null should be == to '' is a bigger question than this issue, but an interesting one to consider in relation to #796

This behavior causes a breaking change between Mapnik 2.1.x and 2.2.x and makes using stylesheets between them difficult since [attr] != null does not work in 2.1.x and older.

Reported downstream at http://support.mapbox.com/discussions/tilemill/6202-tilemill-bug-on-null-postgis-values-after-update-to-v0101-59 from @cquest

So, need to figure out if adding back(wards) compatibility with [attr] != '' matching nulls is needed.

[gravitystorm]

Yes, I found that with 2.2.0-pre - I needed to rewrite many of my stylesheets, since they relied on the foo != '' syntax to mean non-null values in postgis results. Now, foo != '' also matches both non-empty strings and also null values, which leads to features being unexpectedly symbolized.

At the least, there should be something in the release notes for 2.2.0 to say that stylesheets will need a minor reworking. Sorry for not reporting this when I first ran into it!

[springmeyer]

@gravitystorm - thanks for the follow up. So, are you happy with the change and the ability to now filter out nulls by doing foo != null? I presume you fixed your stylesheets by doing foo != null and foo != '' instead of just foo != ''?

As far as API changes like this, I am collecting them at https://github.com/mapnik/mapnik/wiki/API-changes-between-v2.1-and-v2.2. I will add this one shortly.

[cquest]

I'm not really happy with this kind of change. It breaks too many existing stylesheets from my point of view.

[springmeyer]

Okay, @cquest I have added back compatibility to master/2.2.0 release. So, [attr]!= '' should now (again) like 2.1.x and older match both nulls and empty strings. This was done in the pull at #1880 and landed in master/2.2.0 in fdd9afd

[springmeyer]

@cquest - what operating system are you on? (so I can get you a new TileMill dev build to test).

Also, @gravitystorm - I notice there are still a few usages of [attr]!= '' in the openstreetmap-carto stylesheets. This change should allow those stylesheets to work with both 2.1 and the upcoming 2.2. Let me know asap if you find that is not the case.

[springmeyer]

fyi: https://github.com/mapnik/mapnik/wiki/Api-changes-between-v2.1-and-v2.2/_compare/4c0c96e612e3ece130bed91b3fec636ebf1aabf3...5123b428510ae704369bed7b438df9d3303b7113

[cquest]

Tanks a lot for the fix. I'm using TileMill on OSX and will be happy to test the new version.

[springmeyer]

@cquest - please find an OS X dev build of TileMill (using mapnik latest @ c6da53a) at: http://tilemill.s3.amazonaws.com/dev/TileMill-v0.10.1-78-gbb8c6ce.zip

[springmeyer]

fyi @gravitystorm and @cquest - based on the mis-rendering of the openstreets-dc stylesheet even with this build I presume mapbox/carto#247 is still somehow present.

[cquest]

TileMill-v0.10.1-78-gbb8c6ce.zip looks fine with my OSM-FR tilemill project.

Thanks for the fix !

[springmeyer]

Great. Thanks for testing!

On Jun 5, 2013, at 1:21 AM, cquest notifications@github.com wrote:

TileMill-v0.10.1-78-gbb8c6ce.zip looks fine with my OSM-FR tilemill project.

Thanks for the fix !

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