It would be helpful, if parentId could be referenced in Lookup's SOQL filter.
My use case:
We have Account, Opportunity and Document custom object with both Account and Opportunity lookups.
I want to display Documents related list on Account and in the Document's Opportunity lookup only select opportunities of this particular Account.
Currently, the standard field's lookup filter doesn't work, so it's showing all recently viewed opportunities.
My SOQL Filter in this case would be "AccountId = PARENTID"