How can I handle many-to-many relationships in a RESTful API?

Question

Imagine you have two entities, Player and Team, where players can be on multiple teams. In my data model, I have a table for each entity, and a join table to maintain the relationships. Hibernate is fine at handling this, but how might I expose this relationship in a RESTful API?

I can think of a couple of ways. First, I might have each entity contain a list of the other, so a Player object would have a list of Teams it belongs to, and each Team object would have a list of Players that belong to it. So to add a Player to a Team, you would just POST the player's representation to an endpoint, something like POST /player or POST /team with the appropriate object as the payload of the request. This seems the most "RESTful" to me, but feels a little weird.

/api/team/0:

{
    name: 'Boston Celtics',
    logo: '/img/Celtics.png',
    players: [
        '/api/player/20',
        '/api/player/5',
        '/api/player/34'
    ]
}

/api/player/20:

{
    pk: 20,
    name: 'Ray Allen',
    birth: '1975-07-20T02:00:00Z',
    team: '/api/team/0'
}

The other way I can think of to do this would be to expose the relationship as a resource in its own right. So to see a list of all the players on a given team, you might do a GET /playerteam/team/{id} or something like that and get back a list of PlayerTeam entities. To add a player to a team, POST /playerteam with an appropriately built PlayerTeam entity as the payload.

/api/team/0:

{
    name: 'Boston Celtics',
    logo: '/img/Celtics.png'
}

/api/player/20:

{
    pk: 20,
    name: 'Ray Allen',
    birth: '1975-07-20T02:00:00Z',
    team: '/api/team/0'
}

/api/player/team/0/:

[
    '/api/player/20',
    '/api/player/5',
    '/api/player/34'
]

What is the best practice for this?

Can you update the accepted answer, there is another answer with double up votes also by now you may have a better experience to judge the pros/cons of the accepted answer. — Suraj Jain

Donal Fellows Donal Fellows · Accepted Answer · 2011-06-13T08:30:15

In a RESTful interface, you can return documents that describe the relationships between resources by encoding those relationships as links. Thus, a team can be said to have a document resource (/team/{id}/players) that is a list of links to players (/player/{id}) on the team, and a player can have a document resource (/player/{id}/teams) that is a list of links to teams that the player is a member of. Nice and symmetric. You can the map operations on that list easily enough, even giving a relationship its own IDs (arguably they'd have two IDs, depending on whether you're thinking about the relationship team-first or player-first) if that makes things easier. The only tricky bit is that you've got to remember to delete the relationship from the other end as well if you delete it from one end, but rigorously handling this by using an underlying data model and then having the REST interface be a view of that model is going to make that easier.

Relationship IDs probably ought to be based on UUIDs or something equally long and random, irrespective of whatever type of IDs you use for teams and players. That will let you use the same UUID as the ID component for each end of the relationship without worrying about collisions (small integers do not have that advantage). If these membership relationships have any properties other than the bare fact that they relate a player and a team in a bidirectional fashion, they should have their own identity that is independent of both players and teams; a GET on the player»team view (/player/{playerID}/teams/{teamID}) could then do an HTTP redirect to the bidirectional view (/memberships/{uuid}).

I recommend writing links in any XML documents you return (if you happen to be producing XML of course) using XLink xlink:href attributes.

How can I handle many-to-many relationships in a RESTful API?

7 Answers