TYPO3 Core

If I have a field defined like:

'mitglieder_ag' => array(
            'exclude' => 1,
            'label' => 'LLL:EXT:cronmm_ratsinfo/locallang_db.xml:tx_cronmmratsinfo_gremium.mitglieder_ag',
            'config' => array(
                'type' => 'group',
                'internal_type' => 'db',
                'allowed' => 'tx_cronmmratsinfo_ausschussgemeinschaft',
                'size' => 20,
                'minitems' => 0,
                'maxitems' => 100,
                'multiple' => 1,
                'selectedListStyle' => 'width:100px',
                'MM' => 'tx_cronmmratsinfo_gremium_mitglieder_ag_mm',
            )
        ),

that is with "MM=1" and "multiple=1", I am able to have a list where the same record occurs multiple times, e.g:

uid_foreign=45, sorting=1
uid_foreign=44, sorting=2
uid_foreign=44, sorting=3

After moving the entry 45 down one item (to get the list "44, 45, 44"), and I save the record, I end up with the following list:

45, sorting = 1
44, sorting = 3
44, sorting = 3

So not the desired sorting. I cannot separate the same uids: they will end up always being sorted in a "block".

The problem comes from t3lib_loaddbgroup::writeMM(), which cannot handle "multiple=1".

oldMMs:
45, 44, 44

the new itemArray in t3lib_loaddbgroup::writeMM():
44, 45, 44

In t3lib_loaddbgroup::writeMM() TYPO3 will execute the following UPDATEs:

UPDATE tx_cronmmratsinfo_gremium_mitglieder_ag_mm SET sorting = 1 WHERE uid_local=1026 AND uid_foreign=44
UPDATE tx_cronmmratsinfo_gremium_mitglieder_ag_mm SET sorting = 2 WHERE uid_local=1026 AND uid_foreign=45
UPDATE tx_cronmmratsinfo_gremium_mitglieder_ag_mm SET sorting = 3 WHERE uid_local=1026 AND uid_foreign=44

Notice that the last UPDATE will override what the first one did, so that we end up with the MM-table containing:

45, sorting = 2
44, sorting = 3
44, sorting = 3

and we would expect:

44, sorting = 1
45, sorting = 2
44, sorting = 3

Solution for "multiple=1" is to delete all records before we start and just write the new ones in the order they come in (with INSERT).
(issue imported from #M8279)