For each child relations, the subform is visible.
Each attribute of the children has a tool button option to define to which
aggregate the specified value should be compared. This allows for searching
things like
* Each city where the highest building is more than 300 m
* Each sensor where the median value is lower than 50 ppm
* Each feature with a child with a missing value
* ...
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)
