I have implemented form field for collection with allow_delete = true, allow_add = true and by_reference = false options.

I added needed js code to delete elements and append forms for new objects of this collection from prototype.

This works as I expected, but when I submit a few subforms for this collection with indexes not in order (for example 0, 1, 3 instead of 0, 1, 2), then errors from subform with index 3 are assigned to index 2 (which not exists), but subforms are rendered with indexes 0, 1, 3 (same as before submit) after invalid form submit...

I'm sure that it worked correctly in Symfony (errors from subform with index 3 are assigned to subform with same index).