Trigger on update child_index (ordered collection) can be confusing

[jamoore]

The triggers installed to prevent weirdness with Hibernate ordered collections updates (#1056) pose their own weirdness when working from psql:

JcbProduction=> select * from groupexperimentermap where child = 952;
  id  | permissions | version | child | creation_id | external_id | group_id | owner_id | update_id | parent | child_index 
------+-------------+---------+-------+-------------+-------------+----------+----------+-----------+--------+-------------
 1208 |          -1 |       0 |   952 |     5592188 |             |        0 |        0 |   5592188 |      1 |           2
 1207 |          -1 |       0 |   952 |     5592188 |             |        0 |        0 |   5592188 |      0 |           0
 1206 |          -1 |       0 |   952 |     5592188 |             |        0 |        0 |   5592188 |   1052 |           1
(3 rows)

omero => delete from groupexperimentermap where id = 1207;
DELETE 1
omero=> update groupexperimentermap set child_index = child_index - 1 where child = 952;
NOTICE:  Remapping groupexperimentermap 1206 via (-1 - oldvalue )
UPDATE 1
omero=> \q

Perhaps a function which users could use set_default() for example, would be useful.

[jmoore]

Solved as part of #2547.