I'm afraid that it'll not work in my case - objects of family A are objects for placing in world and are created at runtime. Example with building a wall:
1. Object "brick" from family "Placeable" is picked by clicking a button and created in layout.
2. "brick" is placed on ground and it changes it's variable "placed" to true.
3. Another "brick" is picked (and created at runtime) - now, i want to place it on top of previous "brick" (in my case, when they're overlapping, so it's like welding) System correctly detects when they're overlapping, but it doesn't create revolute joint to this previous brick.
I've thought about having another family/container with only "placed" objects and checking against it, but it will need possibility to add objects to family/container at runtime - and right now it's impossible, am i right?
(That was only a exmaple, in my case i need a limited revolute joint)