SELECT issue with references to different tables

Hi,

I have a query I cannot figure out in postgres or actually in any other way than using the client front end, which I
wouldprefer not to do. 

So, I have 4 tables

pets
persons
companies
pets_reference

pets have owners, the owner at any point in time is either a persons or a company, never both at the same time.

So, the pets_reference table has the fields:

refid_pets        matching table pets, field id
refid_persons    matching table persons, field id
refid_companies    matching table companies, field id
ownersince        which is a timestamp

A pet owner can change to persons A, resulting in a record in pets_reference connecting pet and person with a
timestamp,setting refid_companies to zero and refid_persons to person A's record's id value. If the owner changes to
someother person B, then another record is added to pets_reference. Or if the owner for that pet changes to a company,
thena new record is added with refid_persons being zero and refid_companies being the id value of that companies id
fieldvalue. So at the end of the day pets_reference results in a history of owners. 

Now, the problem is with displaying a table with pets and only their current owners. I can't figure out two things.
For one it seems I would need to somehow build a query which uses an if-then branch to check if companies is zero or
personsis zero to ensure to either reference a persons or a companies record. 
The second issue is that I only need the max(ownersince) record, because I only need the current owner and not past
owners.

I toyed around with DISTINCT max(ownersince) and GROUP BY, but it only results in errors. I am not the SQL guru, I know
myway around so far and am learning, but this is kind of another league and I can't really show any good results I've
comeup with so far. Please, can someone help? 

Thanks
Alex