[tolga burcak]"When I add comp_0 into myComp timeline, I can see it. But when I try to change something inside comp_0 with script, it returns undefined. comp_0 acts like an AV Layer, not as a composition. I can change its property (scale) but I can't reach layers in it.
When you use a comp's layer() method, it returns a layer by design; a comp is not a group that exposes its internal members to the containing comp. If you want access to the project item which is used by the layer, you need to use the AVLayer's source() method:
This will return a CompItem. Of course, you create an object to refer to this CompItem:
ThatOtherComp = MyComp.layer("comp_0").source;
But if you're adding comp_0 to the composition programmatically in your script, you may already have an object you can use.