Model a one-to-many with one partition
A parent and its children share a partition key, so one Query fetches them together.
Why
A playlist has many tracks. Instead of a join, both live in one item collection: the playlist's META record and its TRACK items all share the partition key PLAYLIST#chill.
So 'the playlist and everything in it' is a single Query. The relationship was resolved at write time, by where you put the items, not at read time by joining them.
Co-locate a parent and its children under one partition key, and the join disappears into a single Query.
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.
One Query on PLAYLIST#chill returns the playlist record and every track under it.
Run an operation to see the raw engine response.