查看和实体框架数据不对？

Question

I have a view from some table when I select from the view in SQL Server Management Studio it works fine, but when I use Entity Framework to get the data from view it's different.

ReturnDbForTesEntities1 db = new ReturnDbForTesEntities1();
List<VJOBS2> list = new List<VJOBS2>();
list = db.VJOBS2.ToList();

Same number of records but last 2 rows are different.

I have table for job applicant applicant can apply for 2 jobs or more

ApplicantId    ApplicantName  JobId  JobName
   1              Mohamed       1    Developer
   1              Mohamed       2    IT Supporter 

but in list

ApplicantId    ApplicantName  JobId  JobName
   1              Mohamed       1    Developer
   1              Mohamed       1    Developer

Answer 1

There is a subtle problem with views when used from Entity Framework.

If you have a table, do use it with EF, you need to have a primary key to uniquely identify each row. Typically, that's a single column, eg an ID or something like that.

With a view, you don't have the concept of a "primary key" - the view just contains some columns from some tables.

So when EF maps a view, it cannot find a primary key - and therefore, it will use all non-nullable columns from the view as "substitute" primary key.

I don't know what these are in your case - you should be able to tell from the .edmx model.

Let's assume that (ApplicantId, ApplicantName) are the two non-nullable columns that EF now uses as a "substitute" primary key. When EF goes to read the data, it will read the first line (1, Mohamed, 1, Developer) and create an object for that.

When EF reads the second line (1, Mohamed, 2, IT-Supporter) , it notices that the "primary key" (1, Mohamed) is the same as before - so it doesn't bother creating a new object with those values read, but the primary key is the same, it hence must be the same object as it has already read before, so it uses that object instead.

So the problem really is that you can't have explicit primary keys on a view.

Either you can tweak your EF model to make it clear to EF that eg (ApplicantId, JobId) is really the primary key (you need to make sure those columns are both non-nullable ) - or you need to add something like a "artificial" primary key to your view:

CREATE VIEW dbo.VJOBS2
AS
   SELECT 
       ApplicantId, ApplicantName, JobId, JobName,
       RowNum = ROW_NUMBER() OVER(ORDER BY JobId)
   FROM
       dbo.YourBaseTable

By adding this RowNum column to your view, which just numbers the rows 1, 2, ...., n , you get a new, non-nullable column which EF will include into the "substitute PK" and since those numbers are sequential, no two rows will have the same "PK" values and therefore none will erroneously be replaced by something that's been read from the database already.

Answer 2

FYI, I had to add ISNULL to get it to work for me, see the modification in the first line of this code example:

SELECT ISNULL(ROW_NUMBER() OVER(ORDER BY a.OrderItemID),0) as ident, a.* 
FROM
(
      SELECT e.AssignedMachineID, e.StartDate, e.OrderItemID, e2.OrderItemID AS doubleBookedEventID, e.StartTime, e.EndTime, e2.StartTime AS doubleBookedStartDateTime, e2.EndTime AS doubleBookedEndDateTime, DATEDIFF(MINUTE,e2.StartTime,e.EndTime) AS doubleBookedMinutes
  FROM schedule e
    INNER JOIN schedule e2
      ON e.AssignedMachineID = e2.AssignedMachineID
      and e.StartDate=e2.StartDate
      AND e.schedID <> e2.schedID
      AND e2.StartTime BETWEEN DATEADD(minute,1,e.StartTime) AND DateAdd(minute,-1,e.EndTime) where  Coalesce(e.ManuallyOverrided,0)=0 and Coalesce(e.AssignedMachineID,0) > 0
      ) a