对使用OrderBy的linq查询进行分页

Question

我想返回由某个属性分组的某个实体的列表，按时间戳降序排序并进行分页(使用Skip和Take)。我得到的是:

container.CoinMessageSet.Where(
                c => c.MessageState != MessageStateType.Closed &&
                     (c.DonorOperator.OperatorCode.Equals("opcode") ||
                      c.RecipientOperator.OperatorCode.Equals("opcode"))
                ).OrderByDescending(c => c.TimeStamp)
                 .GroupBy(c => c.Reference).Skip(x).Take(100);

在执行时，我得到了异常:

The method 'Skip' is only supported for sorted input in LINQ to Entities. 
The method 'OrderBy' must be called before the method 'Skip'.

…我调用了OrderBy()(尽管是降序)，并且在Skip()之前调用了它!我错过了什么?

Answer 1

您没有对分组进行排序;你需要这样做，然后才能呼出。例如:

.GroupBy(c => c.Reference).OrderBy(grp => grp.Key).Skip(x).Take(100);

(如果您希望组的顺序相反，也可以替换OrderByDescending)

Also:因为你在分组，原始数据中的顺序在很大程度上是没有意义的;你可以把OrderByDescending(c => c.TimeStamp)去掉

所以最终结果:

var query = container.CoinMessageSet.Where(
            c => c.MessageState != MessageStateType.Closed &&
                 (c.DonorOperator.OperatorCode.Equals("opcode") ||
                  c.RecipientOperator.OperatorCode.Equals("opcode"))
            ).GroupBy(c => c.Reference).OrderBy(grp => grp.Key)
             .Skip(x).Take(100);

或:

var query = (from c in container.CoinMessageSet
             where c.MessageState != MessageStateType.Closed &&
                  (c.DonorOperator.OperatorCode == "opcode" ||
                   c.RecipientOperator.OperatorCode == "opcode")
             group c by c.Reference into grp
             orderby grp.Key
             select grp).Skip(x).Take(100);

Answer 2

很可能是GroupBy在OrderByDescending之后。

我想你可以试试:

container.CoinMessageSet.Where(
                c => c.MessageState != MessageStateType.Closed &&
                     (c.DonorOperator.OperatorCode.Equals("opcode") ||
                      c.RecipientOperator.OperatorCode.Equals("opcode"))
                ).OrderByDescending(c => c.TimeStamp)
                 .GroupBy(c => c.Reference).OrderByDescending(c = > c.Key).Skip(x).Take(100);