实体框架0对1关系-未设置对象引用

Question

Having a bit of trouble having not long started with Entity Framework.

I have two tables which have a 0 to 1 relationship. When I select a row from the main table (staff) it's fine unless I select a row that has no joining record in the second table (status). If I do this, then it throws up 'Object reference not set to instance of an object' when it's trying to access the proprty of the second table:

        If Not cls.STATUS_DESC.STAFF_INFO Is DBNull.Value Then
            lblStatusDescription.Text = cls.STATUS_DESC.STAFF_INFO
        End If

The LINQ I use to get the record is:

    Dim account As STAFF =
        (From a In sa.STAFFs
         Where a.STAFF_NO = staffno
         Select a).FirstOrDefault

There is no direct reference to the sub table in the statement, however the join is defined in the database diagram which allows me to reference the properties.

I'm certain it's a very basic issue, but like I said I've only just started using it!

Answer 1

I'm not sure if I can interpret your code correctly but I think it has to do with the Lazy Loading feature of Entity Framework. You must explicitly include the reference to load it into memory. You can to this by using the Include() method as below. I assume that STATUS_DESC is the name of the navigation property. Replace it with the actual one if I'm wrong:

Dim account As STAFF =
    (From a In sa.STAFFs Where a.STAFF_NO = staffno Select a) _
    .Include("STATUS_DESC") _
    .FirstOrDefault

Answer 2

Got around it by adding:

If Not cls.STATUS_DESC Is Nothing Then

end if

to the property call, which seems fairly obvious now I think about it. Is this the most efficient way though, I would have thought that EF would have been able to handle a simple left join.