# Join

A helper for managing bidirectional relations between keys.

Source: docs/source/concepts/join.md
Packages: atom.io
Related: selector-family, atom-family, transaction

`join` manages a bidirectional relation between two sets of keys.

It is useful when either side of a relationship needs to answer questions about
the other side, such as tracks in a playlist and playlists that include a track.

Use `join` when maintaining both directions manually would make the state model
harder to trust.