RFC: Finally support composite primary keys?

[eileencodes (Eileen M. Uchitelle)]

@Eric_Hayes

Basically, in order to have a primary key (or any unique constraint really) on a partitioned table, the partition key (eg tenant_id) must be a part of that key. Seems to me like adding composite keys in ActiveRecord would also cover this use case.

This is one of the reasons we’re prioritizing this project at Shopify. The idea was that we can add composite keys for “real” composite key use cases but that it will also be useful for “virtual” composite keys when we need to include a tenant_id in queries for sharding. My hope is that this will improve Rails integration for Vitess and any system that uses partition key based sharding.