EF为什么在这里抛出异常？

Question

Ok i'm going nuts here, I have been trying to write myself an update method in my repository and for some strange reason I get an exception when I call this ...

public virtual T Update(T entity)
{
    T dbEntity = _context.Set<T>().Find(entity.GetId());

    if (dbEntity != null)
    {
        _context.SaveChanges();
    }

    return dbEntity;
}

The exception occurs on "_context.SaveChanges();", context in this case is a DbContext instance as opposed to an object context. I'm using EF code first and the of T is as follows ...

public partial class AC_Programme
{
    [Key]
    public int Id { get; set; }

    public int OrganisationId { get; set; }

    [Required]
    [StringLength(255)]
    public string Name { get; set; }

    public string Description { get; set; }

    public DateTime Start { get; set; }

    public DateTime End { get; set; }

    public bool Active { get; set; }

    [DataType(DataType.Time)]
    public DateTime AutoCalculationTime { get; set; }

    [DataType(DataType.Time)]
    public DateTime AutoCutoffTime { get; set; }

    public virtual ICollection<AC_Fund> AC_Funds { get; set; }

    public virtual CX_Organisation Organisation { get; set; }
}

I can't understand why despite having done literally nothing, all I did was retrieve an object by id and call save changes, that it feels the need to go messing with relationships.

Any Ideas?

EDIT: oops I forgot to include the exception ...

An exception of type 'System.InvalidOperationException' occurred in EntityFramework.dll but was not handled in user code

Additional information: The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

It gets even more peculiar ...

    public virtual T Update(T entity)
    {
       _context.SaveChanges();
        return null;
    }

... results in the same error.

Answer 1

Looking at the exception I'm assuming you forgot to set the value of a property which is part of a foreign key constraint; maybe OrganizationId ?

Another possibility is that you removed a child entity from AC_Funds property and did not mark the child as deleted. This would result in an orphaned child which means setting the foreign key property to a null value.

Since your foreign key does not allow nulls the child entity cannot be orphaned and must be deleted.

I would suggest opening the Sql Server Profiler and peeking at the update statements generated by Entity Framework.

Answer 2

The problem was that some code further up the stack was adding changes in to EF that should not have been there causing this problem.

Diagnosing this problem was possible by putting a breakpoint on the SaveChanges call and examining "_context.ChangeTracker.Entries()" for changes then looking through the call stack to see what was going on.

Depending on the complexity of your own code if faced with this you may find it beneficial to add the change tracker to the watch window then step through the code until a change appears.

Thanks for the advice @JohnnyHK