How to set up schema and dataset with one to many relationship

We have a customer dataset and dataflow already set up. The data structure is set up to accommodate a one to one key-value pair; one key and one value such as gender (key) = male (value). We are looking to ingest data into RTCDP with a one-to-many relationship; i.e. one key with multiple values.

Sample use case:

We want to set up a key called “Products held”. There is no set number of products held for each customer. A customer could have 1 product held, or 10 or up to 100; it’s different for each customer.

E.g.

Customer001: products_held = product1; he has a single product held.

Customer002: products_held = product5, product15, product23…and so on; he has 16 products held.

Customer003: products_held = product4, product18, product30…and so on; he has 50 products held.

I assume that we will need a new schema for this and perhaps a new dataset. Please advise as to how this can be set up. If there is documentation on this, please let me know.

Thanks!