This adds the possibility to manage data on a normalized relational
database in N:M relations. On the relation editor in a form, the tools
to add, delete, link and unlink work (also) on the linking table if a
relation is visualized as N:M relation.
Configuration is done through the fields tab where on the relation a
second relation can be chosen (if there is a suitable relation in terms
of a second relation on the linking table).
Limitations
===========
QGIS is not a database management system.
It is based on assumptions about the underlying database system. In
particular it expects
* A `ON DELETE CASCADE` or similar measure on the second relation
* Does not take care of setting the primary key when adding features.
Either users need to be instructed to set them manually or - if it's a
database derived value - the layers need to be in transaction mode
(currently only activatable through the API)
- Allow simple operators as supported by OGR (+, -, * )
- Only return a partial compilation success if expression contains
"column"="column2". Since OGR SQL performs case insensitive
comparisons, we need to double check the result using QGIS'
expressions to ensure that case matches. Add unit tests for this
to provider tests.
The postgres provider is modified so that layers with
TIN, PolyhedralSurface and Triangle geometries can be loaded.
Geometries are converted to MultiPolygons (and Polygons for Triangles).
The postgres test is completed to cover the loading of different types
of layers
Failure was caused by GDAL bug #6020, which has now been fixed
in GDAL. It turns out the values are only incorrect if the first
feature in the file has a null value, so I've rearranged the
features in the reference shapefile to avoid this.
