A circular reference was detected while serializing an object of type?

DB MetersTree TABLE

id       text   parentId    state

 0       root          0     open
 1    level 1          1     open
 2    level 1          1     open
 ...     

CONTROLLER

public ActionResult GetDemoTree()
{
    OsosPlus2DbEntities entity = new OsosPlus2DbEntities();
    MetersTree meterTree = entity.MetersTree.FirstOrDefault();

    return Json(meterTree, JsonRequestBehavior.AllowGet);
}

DATA FORMAT THAT SHOULD BE (for example)

[{  
    "id": 1,  
    "text": "Node 1",  
    "state": "closed",  
    "children": [{  
        "id": 11,  
        "text": "Node 11"  
    },{  
        "id": 12,  
        "text": "Node 12"  
    }]  
},{  
    "id": 2,  
    "text": "Node 2",  
    "state": "closed"  
}]  

How can I create tree Json Data? If I write MetersTree with its relationships I get the error that is defined in the title.

I solved it like this:

VIEW MODEL

public class MetersTreeViewModel
{
    public int id { get; set; }
    public string text { get; set; }
    public string state { get; set; }
    public bool @checked { get; set; }
    public string attributes { get; set; }
    public List<MetersTreeViewModel> children { get; set; }
}

CONTROLLER

public ActionResult GetMetersTree()
{
    MetersTree meterTreeFromDb = entity.MetersTree.SingleOrDefault(x => x.sno == 5); //in my db this is the root.
    List<MetersTreeViewModel> metersTreeToView = buildTree(meterTreeFromDb.Children).ToList();

    return Json(metersTreeToView, JsonRequestBehavior.AllowGet);
}

BuildTree Method

private List<MetersTreeViewModel> BuildTree(IEnumerable<MetersTree> treeFromDb)
{
    List<MetersTreeViewModel> metersTreeNodes = new List<MetersTreeViewModel>();
    foreach (var node in treeFromDb)
    {
        if (node.Children.Any())
        {
            metersTreeNodes.Add(new MetersTreeViewModel
            {
                id = node.sno,
                text = node.Text,
                state = node.Text,
                children = BuildTree(node.Children)
            });
        }
        else {
            metersTreeNodes.Add(new MetersTreeViewModel
            {
                id = node.sno,
                text = node.Text,
                state = node.Text
            });
        }
    }

    return metersTreeNodes;
}

Thanks to all who are interested in ...