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?

Answer 1

Make a separate set of /memberships/ resources.

1. REST is about making evolvable systems if nothing else. At this moment, you may only care that a given player is on a given team, but at some point in the future, you will want to annotate that relationship with more data: how long they've been on that team, who referred them to that team, who their coach is/was while on that team, etc etc.
2. REST depends on caching for efficiency, which requires some consideration for cache atomicity and invalidation. If you POST a new entity to /teams/3/players/ that list will be invalidated, but you don't want the alternate URL /players/5/teams/ to remain cached. Yes, different caches will have copies of each list with different ages, and there's not much we can do about that, but we can at least minimize the confusion for the user POST'ing the update by limiting the number of entities we need to invalidate in their client's local cache to one and only one at /memberships/98745 (see Helland's discussion of "alternate indices" in Life beyond Distributed Transactions for a more detailed discussion).
3. You could implement the above 2 points by simply choosing /players/5/teams or /teams/3/players (but not both). Let's assume the former. At some point, however, you will want to reserve /players/5/teams/ for a list of current memberships, and yet be able to refer to past memberships somewhere. Make /players/5/memberships/ a list of hyperlinks to /memberships/{id}/ resources, and then you can add /players/5/past_memberships/ when you like, without having to break everyone's bookmarks for the individual membership resources. This is a general concept; I'm sure you can imagine other similar futures which are more applicable to your specific case.

Answer 2

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.

Answer 3

I would map such relationship with sub-resources, general design/traversal would then be:

# team resource
/teams/{teamId}

# players resource
/players/{playerId}

# teams/players subresource
/teams/{teamId}/players/{playerId}

In RESTful-terms it helps a lot in not thinking of SQL and joins, but more into collections, sub-collections and traversal.

Some examples:

# getting player 3 who is on team 1
# or simply checking whether player 3 is on that team (200 vs. 404)
GET /teams/1/players/3

# getting player 3 who is also on team 3
GET /teams/3/players/3

# adding player 3 also to team 2
PUT /teams/2/players/3

# getting all teams of player 3
GET /players/3/teams

# withdraw player 3 from team 1 (appeared drunk before match)
DELETE /teams/1/players/3

# team 1 found a replacement, who is not registered in league yet
POST /players
# from payload you get back the id, now place it officially to team 1
PUT /teams/1/players/44

As you see, I don't use POST for placing players to teams, but PUT, which handles your n:n relationship of players and teams better.

Answer 4

My preferred solution is to create three resources: Players, Teams and TeamsPlayers.

So, to get all the players of a team, just go to Teams resource and get all its players by calling GET /Teams/{teamId}/Players.

On the other hand, to get all the teams a player has played, get the Teams resource within the Players. Call GET /Players/{playerId}/Teams.

And, to get the many-to-many relationship call GET /Players/{playerId}/TeamsPlayers or GET /Teams/{teamId}/TeamsPlayers.

Note that, in this solution, when you call GET /Players/{playerId}/Teams, you get an array of Teams resources, that is exactly the same resource you get when you call GET /Teams/{teamId}. The reverse follows the same principle, you get an array of Players resources when call GET /Teams/{teamId}/Players.

In either calls, no information about the relationship is returned. For example, no contractStartDate is returned, because the resource returned has no info about the relationship, only about its own resource.

To deal with the n-n relationship, call either GET /Players/{playerId}/TeamsPlayers or GET /Teams/{teamId}/TeamsPlayers. These calls return the exactly resource, TeamsPlayers.

This TeamsPlayers resource has id, playerId, teamId attributes, as well as some others to describe the relationship. Also, it has the methods necessary to deal with them. GET, POST, PUT, DELETE etc that will return, include, update, remove the relationship resource.

The TeamsPlayers resource implements some queries, like GET /TeamsPlayers?player={playerId} to return all TeamsPlayers relationships the player identified by {playerId} has. Following the same idea, use GET /TeamsPlayers?team={teamId} to return all the TeamsPlayers that have played in the {teamId} team. In either GET call, the resource TeamsPlayers is returned. All the data related to the relationship is returned.

When calling GET /Players/{playerId}/Teams (or GET /Teams/{teamId}/Players), the resource Players (or Teams) calls TeamsPlayers to return the related teams (or players) using a query filter.

GET /Players/{playerId}/Teams works like this:

1. Find all TeamsPlayers that the player has id = playerId. (GET /TeamsPlayers?player={playerId})
2. Loop the returned TeamsPlayers
3. Using the teamId obtained from TeamsPlayers, call GET /Teams/{teamId} and store the returned data
4. After the loop finishes. Return all teams that were got in the loop.

You can use the same algorithm to get all players from a team, when calling GET /Teams/{teamId}/Players, but exchanging teams and players.

My resources would look like this:

/api/Teams/1:
{
    id: 1
    name: 'Vasco da Gama',
    logo: '/img/Vascao.png',
}

/api/Players/10:
{
    id: 10,
    name: 'Roberto Dinamite',
    birth: '1954-04-13T00:00:00Z',
}

/api/TeamsPlayers/100
{
    id: 100,
    playerId: 10,
    teamId: 1,
    contractStartDate: '1971-11-25T00:00:00Z',
}

This solution relies on REST resources only. Although some extra calls may be necessary to get data from players, teams or their relationship, all HTTP methods are easily implemented. POST, PUT, DELETE are simple and straightforward.

Whenever a relationship is created, updated or deleted, both Players and Teams resources are automatically updated.

Answer 5

The existing answers don't explain the roles of consistency and idempotency - which motivate their recommendations of UUIDs/random numbers for IDs and PUT instead of POST.

If we consider the case where we have a simple scenario like "Add a new player to a team", we encounter consistency issues.

Because the player doesn't exist, we need to:

POST /players { "Name": "Murray" } //=> 201 /players/5
POST /teams/1/players/5

However, should the client operation fail after the POST to /players, we've created a player that doesn't belong to a team:

POST /players { "Name": "Murray" } //=> 201 /players/5
// *client failure*
// *client retries naively*
POST /players { "Name": "Murray" } //=> 201 /players/6
POST /teams/1/players/6

Now we have an orphaned duplicate player in /players/5.

To fix this we might write custom recovery code that checks for orphaned players that match some natural key (e.g. Name). This is custom code that needs to be tested, costs more money and time etc etc

To avoid needing custom recovery code, we can implement PUT instead of POST.

From the RFC:

the intent of PUT is idempotent

For an operation to be idempotent, it needs to exclude external data such as server-generated id sequences. This is why people are recommending both PUT and UUIDs for Ids together.

This allows us to rerun both the /players PUT and the /memberships PUT without consequences:

PUT /players/23lkrjrqwlej { "Name": "Murray" } //=> 200 OK
// *client failure*
// *client YOLOs*
PUT /players/23lkrjrqwlej { "Name": "Murray" } //=> 200 OK
PUT /teams/1/players/23lkrjrqwlej

Everything is fine and we didn't need to do anything more than retry for partial failures.

This is more of an addendum to the existing answers but I hope it puts them in context of the bigger picture of just how flexible and reliable ReST can be.

Answer 6

I know that there's an answer marked as accepted for this question, however, here is how we could solve the previously raised issues:

Let's say for PUT

PUT    /membership/{collection}/{instance}/{collection}/{instance}/

As an example, the followings will all result in the same effect without a need for syncing because they are done on a single resource:

PUT    /membership/teams/team1/players/player1/
PUT    /membership/players/player1/teams/team1/

now if we want to update multiple memberships for one team we could do as follows (with proper validations):

PUT    /membership/teams/team1/

{
    membership: [
        {
            teamId: "team1"
            playerId: "player1"
        },
        {
            teamId: "team1"
            playerId: "player2"
        },
        ...
    ]
}