Compound references - one document references another document using multiple fields. 

Example:

We have two tables, articles and comments. The articles are indexed by (author_id, article_id). So the comments reference the articles by (author_id, article_id).

In the storage, we do support this type of relationships. Fields is an array.

type RelationshipSide = {
  ns: Namespace;
  cardinality: number;
  fields: FieldPath[];
};

This task is to make sure these types of relationships can be managed via the Data Modeling side panel, as well as represented in the diagrams.

For reference, see the attached screenshots showing a Composite Foreign Key in the Relational Migrator (sidepanel and diagram).