/// <summary>
/// 获取求和值。
/// </summary>
/// <typeparam name="TModel">当前模型类型。</typeparam>
/// <param name="db">数据库实例。</param>
/// <param name="column">当前列。</param>
/// <param name="expression">条件表达式。</param>
/// <returns>返回当前值。</returns>
public static int Sum <TModel>(this IDbContextBase <TModel> db,
Expression <Func <TModel, object> > column, Expression <Predicate <TModel> > expression = null)
{
return(db.Sum(column, expression, Convert.ToInt32));
}