How to build a CompressedGRangesList with cdsstart/cdsend using custom txdb?

Hello Biostars Community,

How do I build a CompressedGRanges List with cdsstart/cdsend in listData using a custom txdb using GenomicFeatures?

I think this may be a simple GenomicFeatures task, but this is my first time doing this so I am having trouble. I have tried a bunch of times by just guessing-and-checking, but have not had success yet.

Below is an example of how I want my object to look (please see right below listData)

Below is my best attempt (after multiple other tries):

GRList <- transcriptsBy(mm39.txdb, by = "cds")

Help would really be appreciated. Thank you in advance!

-Pratik