In one of our apps I would like to store objects like this:
{ children: [{ id: 'c1' }, { id: 'c1' }] }

I would like to be able to create an index to quickly find all parents for child 'c1'.

Sadly an index like "children.id" { multiEntry: true } does not work here.

I would propose something like "children[].id" { multiEntry: true } to indicate, that the children property should be iterated, not their id field.

Related discussions:
https://social.msdn.microsoft.com/Forums/en-US/dbf2fd2d-a8ca-4228-8a49-3948d44f7e26/indexeddb-and-complex-keypaths-for-index (where Israel Hilerio (MSFT) mentions this should be discussed for IDB v2)
https://github.com/dfahlander/Dexie.js/issues/86