Multiple-field styling? Live updates?

[mroswell]

Is there styling in Carto across multiple data fields?
For instance:
If AgCommittee='Y' and Party="R" color Red
If AgCommittee='Y' and Part='D' color Blue
If Ag Committee is Null color white

I would imagine a syntax such as:

#Congress [AgCommittee='Y' AND Party = 'R'] {
polygon-fill: @red;
}

Or, do I need to pre-process all this in a GIS (by adding a color-by field with entries such as 'AgDem' and AgRepub'?

Related to this, if I edit a file in a GIS, for instance QGIS, is there a way to have the file updates be reflected in the currently opened TileMill.

[tmcw]

Hey @mroswell: sure, what you're looking for is expressed in Carto as

#Congress [AgCommittee='Y'][Party = 'R'] {
polygon-fill: @red;
}

I'm not sure about your cache question offhand - I think there's some control of this, but not sure.

[mroswell]

Thanks very much. I'm guessing there's a performance hit for this multifield styling. Is it significant? (Remember, the Congress file I've got is torturously slow to start with)

[tmcw]

It'll be very, very minor - likely unnoticable. I assume you've done the normal stuff to make your data faster - reprojection & it's in a fast format?

[springmeyer]

re: reflecting changes made in QGIS in TileMill - normally all that should be needed is to save the currently tilemill project. However, there is a bug/feature in mapnik that keeps, specifically, shapefile references in a cache (mapnik/mapnik#784). So, for shapefiles, getting new data in tilemill requires actually re-starting TileMill or renaming the file and reloading it as a new layer. I can understand if this is a major hassle - so we need to fix this upstream in mapnik somehow.

[mroswell]

See related question: https://github.com/mapbox/tilemill/issues/989
(how to convert shapefiles to SQLite format, since that seems to be more workable data source)