@Id private Long listId;
    @Parent private Key<User> userKey;
    private String name;
    private String listPic = "http://localhost/default"; // There must be a default source for pic
    private int memberCount;

    public UserList() {
    }

    public UserList(Key<User> userKey, String name, int memberCount) {
        this.userKey = userKey;
        this.name = name;
        this.memberCount = memberCount;
    }

    public Long getListId() {
        return listId;
    }

    public void setListId(Long listId) {
        this.listId = listId;
    }

    public Key<User> getUserKey() {
        return userKey;
    }

    public void setUserKey(Key<User> userKey) {
        this.userKey = userKey;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getListPic() {
        return listPic;
    }

    public void setListPic(String listPic) {
        this.listPic = listPic;
    }

    public int getMemberCount() {
        return memberCount;
    }

    public void setMemberCount(int memberCount) {
        this.memberCount = memberCount;
    }
}

public List<UserList> getUserList(@Named("userId") Long userId){
    // Get userList where ancestor is userId
    return ofy().load().type(UserList.class).ancestor(Key.create(User.class, userId)).list();
}

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: java.util.HashMap["items"]->$Proxy32[0]->org.octabyte.zeem1.Datastore.UserList["userKey"]->com.googlecode.objectify.Key["root"])