Traverse a relationship both ways
An index that swaps the partition and sort keys reads the same edges from the other side.
Why
A maintainer belongs to many projects, and a project has many maintainers. Each membership is stored once, as an edge item keyed pk = PERSON, sk = PROJECT, carrying a second pair of keys that swap the two: gsi1pk = PROJECT, gsi1sk = PERSON.
So the base table answers 'Ada's projects' and the inverted index answers 'httpkit's maintainers', from the very same edges. One relationship, two directions, nothing duplicated. The profile items carry no index keys, so only the edges appear in the index.
Store a many-to-many as edges, then an inverted index, partition and sort swapped, reads the relationship from either side.
These rows are real and already here. Running an operation against them downloads a DynamoDB-compatible engine, about 850 KB, and executes it in this tab.
On the base table, Ada's partition holds one edge per project she maintains.
Run an operation to see the raw engine response.