Update ParentField.php

[Sindoweb]

Description

Changed parent to parent_id because this gave errors when saving an edit of a page.

Reason for this change

I figured I could change this since all the references in the Concerns are to parent_id.
It solved my issue, but I'm not sure if this change is correct.

[sten]

@Sindoweb thanks for the PR. That would indeed have cleaner to use parent_id.

The problem is that by changing the constant, it will be breaking for all upgrades. So the options are:

1. try to make both parent & parent_id work
2. release a major version

I will contemplate on this choice a bit more. If you have ideas, shoot! ;-)

[Sindoweb]

@sten

1. try to make both parent & parent_id work

Maybe a check if parent_id is not empty (and use that if it isn't).
Otherwise use parent.

I'll play around with this to see what gives.