Dapper join query error in mapping

I am facing issues in below piece of code. Everything looks fine but still I get below Error:

'MetalItems' does not contain a definition for 'metals' and no extension method 'metals' accepting a first argument of type 'MetalItems' could be found (are you missing a using directive or an assembly reference?) E:\VS2010\Jewellery\SilverSpoon\SilverSpoon\API\models\MetalItems.cs 38 52 SilverSpoon

CODE

class MetalItems
{
    public int ID { get; set; }
    public int metal_id { get; set; }
    public string item_name { get; set; }
    public Metals metals {get; set;}

    public IEnumerable<MetalItems> findAllJoin<MetalItems>()
    {
        IEnumerable<MetalItems> data = null;

        using (var sqlConnection = Database.getConnection())
        {
            data = sqlConnection
                .Query<MetalItems, Metals, MetalItems>(
                @"SELECT * FROM metal_items mi JOIN metals m ON mi.metal_id = m.metal_id ORDER BY mi.metal_id",
                                        (mi, m) =>
                                        {
                                            mi.metals = m;
                                            return mi;
                                        }, 
                                        splitOn: "metal_id");
        }
        return data;
    }
}

class Metals
{
    public int metal_id { get; set; }
    public string metal_name { get; set; }
    public decimal rate_per_gram { get; set; }
    public decimal making_charges { get; set; }
}

ERROR LINE is display in below snapshot;