Rendering a map with Mapnik and PostGIS

I’m trying to render a map using Mapnik and PostGIS, I feel I’m close but it doesn’t work yet. Here is what I’ve done so far (it’s the first time I work with a database, so it’s likely that there are stupid mistakes due to the fact that I don’t know how databases work).

I’m working in Arch Linux with Mapnik 2.2.0, PostgreSQL 9.2.4, PostGIS 2.0.3 and osm2pgsql-git 20130512.

I downloaded the montpellier.osm.pbf file from http://metro.teczno.com/#montpellier (I didn’t download any coastline file, for now I’m just trying to render some part in the middle of a city). I created a database with PostGIS enabled and imported the pbf file (using osm2pgsql), using various instructions on the Wiki. I don’t fully understand the various usernames needed for a PostgreSQL database, but I think the main PostgreSQL username is "postgres" and the other username (not sure what this one is for) is my Unix login ("fractal"). Anyway, the import seems to have done something (it took something like an hour to complete) and I have the following:

$ psql --username=postgres --dbname=osm --command="\d+"
                           List of relations
 Schema |        Name        | Type  |  Owner   |  Size   | Description 
--------+--------------------+-------+----------+---------+-------------
 public | geography_columns  | view  | postgres | 0 bytes | 
 public | geometry_columns   | view  | postgres | 0 bytes | 
 public | planet_osm_line    | table | postgres | 32 kB   | 
 public | planet_osm_nodes   | table | postgres | 104 kB  | 
 public | planet_osm_point   | table | postgres | 24 kB   | 
 public | planet_osm_polygon | table | postgres | 24 kB   | 
 public | planet_osm_rels    | table | postgres | 2184 kB | 
 public | planet_osm_roads   | table | postgres | 16 kB   | 
 public | planet_osm_ways    | table | postgres | 141 MB  | 
 public | spatial_ref_sys    | table | postgres | 3216 kB | 
(10 rows)

So it seems that the database has been populated with stuff.

Then I tried to adapt the tutorial at http://wiki.openstreetmap.org/wiki/Mapnik:_Rendering_OSM_XML_data_directly. I downloaded the style file there, made it compatible with Mapnik 2, changed various details, and tried to change the datasource from XML to PostGIS. What I have now is the following (in the file test.xml):

http://pastebin.archlinux.fr/467464

(I looked at the OSM style files to see what kind of parameters are needed for postgis)

Then I have the following Python script (adapted from the other tutorial):

!/usr/bin/env python2

from mapnik import *

mapfile = 'test.xml'
map_output = 'mymap.png'

m = Map(4*1024,4*1024)
load_map(m, mapfile)
bbox=(Envelope( 3.873,43.603,3.878,43.606 ))

m.zoom_to_box(bbox)
print "Scale = " , m.scale()
render_to_file(m, map_output)

(the bounding box is supposed to be a place with several roads)

When I run the script, there are no error and the mymap.png file is created, but it’s completely yellow (the background color in the style file) with no road. Maybe that’s because there is a problem with the connection to the database, but I don’t know how to fix it. I also tried with planet_osm_line instead of planet_osm_roads (I’m not sure where the highways are supposed to be) but it’s the same, and I tried with planet_osm_ways but it failed with the error:

RuntimeError: PostGIS: geometry name lookup failed for table 'planet_osm_ways'. Please manually provide the 'geometry_field' parameter or add an entry in the geometry_columns for 'planet_osm_ways'.

Thanks for your help!