Mapnik TextSymbolizer showing nothing above OpenStreetMap base layer

Question

I'm trying to use Python to create a TextSymbolizer to display a trail name above the trail LineSymbolizer. Below this layer is the OSM base layer which contains its own data for that trail.

I can display the line but I cannot display any text.

I've compiled Mapnik with debug turned on only to find that there's no logging near the text symbolizer code. I had a go with gdb but I haven't spent enough time to work out how to debug from python into the Mapnik shared library. I'm happy enough to debug but I'm not familiar with gdb.

I also edited the osm.xml and took away the trails and trail names which might be interfering with the TextSymbolizer but I didn't see any difference.

Relevant code:

    imgx = 400
    imgy = 400

    m = mapnik.Map(imgx,imgy)
    m.buffer_size = 1000

    workoutParams = dict(host='localhost',port='5432',user='postgres',password='password',dbname='postgis')
    workoutParams['estimate_extent'] = False
    workoutParams['extent']='-180,-89.99,180,89.99'
    workoutTable = "(select id, geometry, 'really long trail name' as trail_name from pub_trail_segment where pub_trail_id = %s) as route" % idNumber
    workoutParams['table']= workoutTable
    workoutDatasource = PostGIS(**workoutParams)
    workoutLayer = Layer('route','+proj=latlong +datum=WGS84')
    workoutLayer.datasource = workoutDatasource
    workoutStyle = mapnik.Style()
    segmentStyleRule = mapnik.Rule()
    segmentStyleRule.symbols.append(mapnik.LineSymbolizer(mapnik.Color('blue'),1)) #50 for print
    segmentTextSymbolizer = mapnik.TextSymbolizer('trail_name', 'book-fonts', 20, mapnik.Color('blue'))
    segmentTextSymbolizer.halo_radius = 1
    segmentTextSymbolizer.label_placement = mapnik.label_placement.LINE_PLACEMENT
    segmentTextSymbolizer.allow_overlap = True
    segmentTextSymbolizer.avoid_edges = False

    segmentStyleRule.symbols.append(segmentTextSymbolizer)
    workoutStyle.rules.append(segmentStyleRule)
    m.append_style('route',workoutStyle)
    workoutLayer.styles.append('route')
    m.layers.append(workoutLayer)
    m.zoom_to_box(bbox)
    im = mapnik.Image(imgx,imgy)
    mapnik.render(m, im)

What am I doing wrong?

Answer 1

Two options: Create a second style object and attach the text symbolizer to that style, then append that second style to the layer. Layers in Mapnik can accept any number of styles and will render them in the order attached, so if you attach the a second style with text rules, then they will be rendered second. However in the case of polygons with text most people go further and create a second layer with its own style containing text rules. This will ensure that all text is rendered after all other geometries are rendered.

Answer 2

The problem was this part in the PostGIS element

'really long trail name' as trail_name

Mapnik appeared to have a problem with it. When I replaced it with the proper join the name displayed.