/// <summary>
/// 指定字段名自减并返回当前语句
/// </summary>
/// <typeparam name="T">实体类类型</typeparam>
/// <param name="cmd">更新语句</param>
/// <param name="expr">实体类属性</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 Boolean UpdateEntity(User user)
/// {
/// return this.Update()
/// .Decrease<User>(c => c.LoginTimes)
/// .Where<User>(c => c.UserID == user.UserID)
/// .Result() > 0;
///
/// //UPDATE tbl_Users SET LoginTimes = LoginTimes - 1 WHERE UserID = @UserID
/// //@UserID = user.UserID
/// }
/// }
/// ]]>
/// </code>
/// </example>
public static UpdateCommand Decrease <T>(this UpdateCommand cmd, Expression <Func <T, Object> > expr)
{
MemberExpression left = ExpressionHelper.GetMemberExpression(expr.Body);
if (left == null)
{
throw new ExpressionInvalidException();
}
DatabaseColumnAttribute attr = ExpressionHelper.GetColumnAttribute(cmd, left);
if (attr == null)
{
throw new NullAttributeException();
}
return(cmd.Decrease(attr.ColumnName));
}