QGIS » QGIS Application

I'm not really sure about how to fix this, and even if that's a bug: the issue is triggered by the symbol renderer which has a data-defined property on symbol size wth an expression that is based on @map_scale property.

But, in case of a getlegendgraphic request, there is no map and there is no map_scale so the expression evaluation obviously fails, in other words, the getlegendgraphic request does not have all the information required to calculate the map_scale and to determine the symbol size.

I'm not sure about the best approach here. I could try to fake the request and inject a map_scale into the context based on server full extent and an arbitrary map size that makes sense on the web (1024x768?).

Another option would be to ignore the data-defined property in case the context does not contain the information required to compute the expression successfully (but this would mean messing up with core routines of symbol drawing and I don't really recommend it, I'd rather prefer a server-only patch to handle this case).

I'll try the server approach first.

Ok, after some more thoughts and an enlightning talk with Nyall, I decided that this is not a bug.

The behavior of expression evaluation is designed to not raise an exception when a variable is undefined but evaluate it to NULL instead.

This means that in the attached test case, the last rule (ELSE) is triggered and it returns a 0 as the symbol size.

The solution is to add an extra case to the expression:

CASE
WHEN @map_scale <= 2000 THEN 9
WHEN @map_scale > 2000 AND @map_scale <= 5000 THEN 8
WHEN @map_scale > 5000 AND @map_scale <= 10000 THEN 7.5
WHEN @map_scale > 10000 AND @map_scale <= 20000 THEN 6.5
WHEN @map_scale > 20000 AND @map_scale <= 100000 THEN 6
WHEN @map_scale > 100000 AND @map_scale <= 200001 THEN 5.5
WHEN @map_scale IS NULL THEN 8
ELSE 0
END