In the Data Warehouse mappings, there is a lookup functionality built-in to perform any code lookups from the landing tables in Compose.
I was looking for similar functionality in the Data Mart ETL too.
We have built dimensions where some of the column are codes only known to the source system (there are thousands of such columns in our model), in the DataMart layer we want to provide meaningful descriptions for those codes by doing a lookup from the code description dimension (which has very static data that doesn't change very often).
One of the ways to implement this is using custom ETL code in Compose, but since we have so many code columns we are looking for a standard lookup functionality in the Data Mart expressions to avoid maintaining a lot of custom code.
Also we want to avoid doing this type of data denormalization in the warehouse also dont want to create relationships for thousands of code columns, this requirement fits well in the Data Mart/BI layer to be built for reporting.