docs: function getRelationship(payload, resource, key) {
    var relationships = resource.relationships && resource.relationships[key];
    if (!relationships || !relationships.data) {
      return;
    }
    return !Array.isArray(relationships.data)
      ? getIncluded(payload, relationships.data.type, relationships.data.id)
      : relationships.data.map(function (relationship) {
        return getIncluded(payload, relationship.type, relationship.id);
      }).filter(function(entity) { return entity });
  }