How do I use a Physics Constraint Component to attach to a bone in a Skeletal Mesh?

Development Programming & Scripting Blueprint
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1

This seems like it should be straightforward since the Physics Constraint Object has a Constraint Bone property for each component. The use case is that I have an object that needs to attach to one of the moving parts of this mesh using physics. However if I set the Bone Name to one of the bones in the Skeletal Mesh, the joint ends up connecting to 0,0,0. (Note that if I leave the bone empty, it connects to the Skeletal Mesh directly, but it won’t follow the bone when it moves. It also works this way if I add an invalid bone name.)

I think I’ve tracked it down to USkeletalMeshComponent::GetBodyInstance. It finds the Body Index in the Physics Asset, but the Bodies array is empty here:

			int32 BodyIndex = PhysicsAsset->FindBodyIndex(BoneName);
			if(Bodies.IsValidIndex(BodyIndex))
			{
				BodyInst = Bodies[BodyIndex];
			}

I can’t find any way to get this to populate, I searched through the code and the only references to the Bodies array are reading from it.

If anyone has had luck using Physics Component to attach to a bone in a skeletal mesh I’d love to hear how!

2

Just to note, I’m very confused that it behaves differently if I specify a valid name vs. an invalid name (or just leave it set to “None”). I’ve stepped through most of the code that seems relevant and all of these paths should just return nullptr. I can’t see where the logic is different between valid name that it can’t find vs. invalid name.