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)
* layerFilterExpression
Return an additional filter, used in
WMS/GetMap, WMS/GetFeatureInfo, WFS/GetFeature to filter the features
* layerFilterSubsetString
Return an additional the subset string (typically SQL) filter.
Faster than the layerFilterExpression but not supported on all the
type of layer
* layerPermissions
Change the rights on the layer per user (known by the plugin)
Concern rights: publish, insert, update, delete.
Mostly used in WFS/Transaction, and the publish in all requests.
* authorizedLayerAttributes
Be able to show some attributes only for a subset of user
Used in: WMS/GetFeatureInfo, WFS/GetFeature
* allowToEdit
Be able to don't allow to edit a particular feature, in our case base
on the Geometry
Used in: WFS/Transaction
* cacheKey
Cache key to used to create the capabilities cache, "" for no cache,
shouldn't contains any "-", default to ""
