How to debug 'Layer is not valid error' from Python plugin?

Question

I'm trying to understand what goes wrong with a QGIS python plugin. Basically I'm trying to render a series of POINT.

The query works fine, at least giving it by hand from inside PostGIS DB:

"SELECT  gid, CAST (gid as text)|| '_' || (ST_DumpPoints(the_geom)).path[2] as key,  (ST_DumpPoints(the_geom)).geom as points FROM ways"

I concat gid and path[2] because after splitting the_geom (MULTILINE) field into several POINT, gid are duplicated and I think they are no longer usable as key (with this trick key should be unique, but it seems not solve my problem).

I tried to create the layer this way:

def createPointLayer(self):
    uri = self.db.getURI()
    query = "SELECT  gid, CAST (gid as text)|| '_' || (ST_DumpPoints(the_geom)).path[2] as key,  (ST_DumpPoints(the_geom)).geom as points FROM ways"

    uri.setDataSource("", "(" + query + ")", "points","","key")
    layerName = "NodeLayer"
    aLayer = self.iface.addVectorLayer(uri.uri(), layerName, self.db.getProviderName())

I think the query works fine (QGIS takes a long time before display the error. I guess something goes wrong after the query has been executed).

When the plugins run QGIS output the following error:

Layer is not valid:

The layer dbname='pgrouting' user='user' password='user' key='gid' table="(SELECT gid, CAST (gid as text)|| '_' || (ST_DumpPoints(the_geom)).path[2] as key, (ST_DumpPoints(the_geom)).geom as points FROM ways)" (points) sql= is not a valid layer and can not be added to the map

I already had to struggle against the Layer is not valid error, but I don't know how to debug it. So I have 2 questions:

1. Could anyone tell me what's my mistake in this specific case?
2. How can I debug 'Layer not valid error'? How can I get more information on which is the problem?

EDIT:

QGIS log console display the following message:

No key field for query given

Answer 1

It seems your "key" field must have type integer and not text. I reproduced your problem with the following snippet:

uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "mydb", "user", "pass")
query="SELECT  id, CAST (id as text)|| '_' || (ST_DumpPoints(geom)).path[2] as key, (ST_DumpPoints(geom)).geom as points FROM line"
uri.setDataSource("", "(" + query + ")", "points", "", "key")
vl = iface.addVectorLayer(uri.uri(), "QueryLayer", "postgres")

Only after I changed the "key" field type to "int", I could get the layer loaded into QGIS:

uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "mydb", "user", "pass")
query="SELECT  id, CAST(CAST (id as text)|| (ST_DumpPoints(geom)).path[2] as int) as key, (ST_DumpPoints(geom)).geom as points FROM line"
uri.setDataSource("", "(" + query + ")", "points", "", "key")
vl = iface.addVectorLayer(uri.uri(), "QueryLayer", "postgres")

Note that I had to adjust your gid to id and your the_geom to geom, in order to work with my own data.

Answer 2

I had the same problem: "The layer ... is not a valid layer and can not be added to the map". It was just because my "id" was not unique (by default, the plugin used the first column as id (called "edge_id", so it took me a bit to figure out) which was not unique, instead of the "id" column, which was unique.