Emmanuel Bernard, Davide D'Alto Working on this, I - somewhat unintentionally - also changed this for assocations. So e.g. instead of

...
"bankAccounts" : [
    { "bankAccounts_id" : "accountXYZ" },
    { "bankAccounts_id" : "accountABC" }
]
...

my local change would persist the following:

...
"bankAccounts" : [
    "accountXYZ",
    "accountABC"
]
...

I guess this reads a bit better, but the mapping would be different for simple and composite keys. I'm inclined to say that it should stay as is for associations and only be changed for element collections. One practical issue we'd otherwise have is that we couldn't tell apart associations and element collections in the count-based tests. WDYT?