How to model social graph in Java

google-app-engine, java, modeling

Solution

We've modeled user relations as a simple UserRelation entity:

class UserRelation {
  User _from;
  User _to;
  RelationState _state;
}

Where RelationState is an enum, describing states (normally, there is more than friendship)

enum RelationState {
  BLOCKED, NONE, PENDING_FRIEND, FRIEND;
}

Actually, we also use this enum for authorizaton, e.g. on user profiles.

enum RelationState implements IRole {
  BLOCKED, NONE(BLOCKED), PENDING_FRIEND(NONE), FRIEND(PENDING_FRIEND);

  private final List<IRole> _impliedRoles;
  private final List<String> _roleStrings;

  private RelationState(final IRole... impliedRoles) {
    HashSet<IRole> set = new HashSet<IRole>();
    for (final IRole impliedRole : impliedRoles) {
      set.add(impliedRole);
      set.addAll(impliedRole.getImpliedRoles());
    }
    _impliedRoles = Collections.unmodifiableList(new ArrayList<IRole>(set));

    ArrayList<String> list = new ArrayList<String>(getImpliedRoles().size() + 1);
    list.add(getName());
    for (final IRole implied : getImpliedRoles()) {
      list.add(implied.getName());
    }
    _roleStrings = Collections.unmodifiableList(list);
  }

  public List<IRole> getImpliedRoles() {
    return _impliedRoles;
  }

  public String getName() {
    return name();
  }

  public boolean hasRole(final IRole role) {
    return this == role || _impliedRoles.contains(role);
  }

  public List<String> getRoleStrings() {
    return _roleStrings;
  }
}

public interface IRole {
  public List<? extends IRole> getImpliedRoles();
  public String getName();
  public boolean hasRole(final IRole role);
  public List<String> getRoleStrings();
}

It's easiest to have two objects for each (symmetric) relationship (e.g. friendship as used on facebook) and only a single object for non-symmetric relationships (e.g. followers as used on twitter or blocked users). While this might look like overhead in the first place, using two objects certainly simplifies querying.

I think the AppEngine part itself should then be pretty straight forward.

Problem

Original source