RelationController.js

server.get('/api/relations', function (req, res, next) {
    var predicates = [];

    if (req.query.type) {
      predicates.push(Predicates.eq('type', req.query.type));
    }

    if (req.query.sourceId) {
      predicates.push(Predicates.eq('sourceId', req.query.sourceId));
    }

    if (req.query.targetId) {
      predicates.push(Predicates.eq('targetId', req.query.targetId));
    }

    if (req.query.eitherId) {
      predicates.push(Predicates.or(
          Predicates.eq('sourceId', req.query.eitherId),
          Predicates.eq('targetId', req.query.eitherId)));
    }

    Relation.filter(function (doc) {
      return Predicates.all(predicates, doc);
    }, function (err, result) {
      if (err) {
        return next(err);
      }
      res.send(result);
    });
  });