/// <summary>
/// 按指定列排序并返回当前语句
/// </summary>
/// <typeparam name="T">实体类类型</typeparam>
/// <param name="cmd">查询语句</param>
/// <param name="expr">实体类属性</param>
/// <param name="orderType">排序类型</param>
/// <exception cref="ExpressionInvalidException">表达式不正确</exception>
/// <exception cref="NullAttributeException">没有设置特性</exception>
/// <returns>当前语句</returns>
/// <example>
/// <code lang="C#">
/// <![CDATA[
/// public class UserRepository : DatabaseTable<User>
/// {
/// //other necessary code
///
/// public List<User> GetAllEntities()
/// {
/// return this.Select()
/// .Querys<User>(c => new { c.UserID, c.UserName })
/// .OrderBy<User>(c => c.UserID, SqlOrderType.Desc)
/// .ToEntityList<User>(this);
///
/// //SELECT UserID, UserName From tbl_Users ORDER BY UserID DESC
/// }
/// }
/// ]]>
/// </code>
/// </example>
public static SelectCommand OrderBy <T>(this SelectCommand cmd, Expression <Func <T, Object> > expr, SqlOrderType orderType)
{
DatabaseColumnAttribute attr = SelectCommandExtension.GetColumnAttribute(cmd, expr.Body);
String tableName = EntityHelper.InternalGetTableName(typeof(T));
return(cmd.OrderBy(tableName, attr.ColumnName, orderType));
}
