[英]Map sql query with EF query doesn't work
我正在嘗試使用EF6執行查詢.sql格式的我的查詢是這樣的:
/****** Script for SelectTopNRows command from SSMS ******/
SELECT
[SUBJECT],COUNT(SUBJECT)
FROM [PGC].[dbo].[QC] group by [SUBJECT]
效果很好。
主題是字符串類型。
但是我將查詢轉換為EF,如您在此處看到的:
listDataSource =
db.QCs.Where(j => j.SUBJECT != null).ToList().GroupBy(i => new {i.SUBJECT}).Select(m => new chart()
{
date = m.Key.SUBJECT,
Count = m.Count(i=>i.SUBJECT).ToString()
}).ToList();
m.Count(i => i.SUBJECT)返回錯誤: can't convert expression type string to return type bool
最好的祝福
您不應該使用Count<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>)
因為它:
返回一個數字,該數字表示指定序列中有多少個元素滿足條件。 資源
i=>i.SUBJECT
是Func<TSource, String>
但是該方法需要Func<TSource, Boolean>
,因此您收到該異常消息。
而是使用Count<TSource>(IEnumerable<TSource>)
,它將返回序列中元素的總數,例如:
Count = m.Count().ToString()
擴展了Yuriy的答案(完全可以接受),並進行了一些修復,主要是出於性能方面的考慮:
listDataSource = db.QCs
.Where(j => j.SUBJECT != null)
.GroupBy(i => i.SUBJECT) //no need to create an anonymous type, you don't use it afterwards
.Select(group => new { SUBJECT = group.Key, Count = group.Count()}) //counting in SQL is faster
.AsEnumerable() //materialization, all the heavy lifting is performed by SQL until now
.Select(value => //seems redundant, but the previous selection just translates into an SQL select statement with SUBJECT and Count columns
new chart
{
date = value.SUBJECT,
Count = value.Count.ToString()
})
.ToList();
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