Simple question:
does an ability damage or an ability drain effect apply to the target of an attack, it the physical damage does not get through the damage reduction of the target?
For example, see the denized of leng monster, or the green hag.
If their claws' phyisical damage does not pierce the dr, does the drain or the weakness effect apply?
And a related question: is there some kind of "stack" order to apply damage to creatures in pathfinder? Does the physical damage always get computed first and the rider effects later?
I believe the general rule is that if no damage is dealt by the attack then any rider effects are also negated, but I'm not sure where it is written. I'm sure someone else can though.
Using the hag as an example, if she is using her claw attacks and the damage is completely negated by DR, then the weakness as a rider affect shouldn't apply.
If the hag uses the standard action touch attack version (which doesn't deal damage and isn't a rider) then DR does nothing.
In general yes, rider effects are applied after physical damage if this is relevant for some reason.
I found the relevant text, in the Special Abilities section of the Core Rulebook.
"Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact."
It seems clear that ability damage and drain is not energy damage, energy drain, touch attack or non-injury poison and disease, so they follow the general rule and do not apply if the dr is not pierced.
Do you agree? I need the most confirmations I can get as one of my mates (in this particular case he was the DM!) is quite stubborn :)
Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.
(emphasis mine)
Your questions:
does an ability damage or an ability drain effect apply to the target of an attack, it the physical damage does not get through the damage reduction of the target?
In general, no, as stated by the other posters, if DR negates the physical damage on a hit than any rider effects are negated as well. But there are exceptions (see above).
For example, see the denized of leng monster, or the green hag.
If their claws' phyisical damage does not pierce the dr, does the drain or the weakness effect apply?
For the denizen of Leng, no, Dexterity Drain would not apply if it couldn't beat DR. For the green hag, however, the Weakness ability can be used as a touch attack; while it normally wouldn't, a hag using the special 2d4 touch attack option would completely ignore DR (even if said attack were delievered with an unarmed strike which then got negated by DR).
And a related question: is there some kind of "stack" order to apply damage to creatures in pathfinder? Does the physical damage always get computed first and the rider effects later?
I don't believe this is explicitly stated anywhere, but in general it's presumed the physical damage is computed first (as demonstrated by how DR works).
Edit: Ninja'd - I'm not seeing anyone here disagreeing on the matter, though (we all have the same general position), so I'd say you have your consensus.