Why does NHibernate delete then insert composite-elements on select?

Question

Can someone explain this little mystery to me about how NHibernate handles composite elements. I have classes that look like this; ``` public class Blog { public virtual int Id { get; private set; } public virtual ISet Comments { get; set; } } public class Comment { public virtual string CommentText { get; set; } public virtual DateTime Date { get; set; } } ``` and mappings like this; ```

``` However when i perform a select like this; ``` using (ITransaction transaction = session.BeginTransaction()) { Blog blog = session.CreateCriteria(typeof(Blog)) .SetFetchMode("Comments", FetchMode.Eager) .Add(Expression.IdE…

Accepted Answer

I think you need to override Equals() and GetHashCode() on Comment. NHibernate doesn't have an ID to go on for entity equality so you have to define what makes a comment entity equal to another comment.

Could be wrong :)

Edit

From nhibernate.info (8.2)

Note: if you define an ISet of composite elements, it is very important to implement Equals() and GetHashCode() correctly.

And an example of implementing Equals / GetHashCode from nhibernate.info (4.3)

```
public class Cat
{    
    ...
    public override bool Equals(object other)
    {
        if (this == other) return true;

Cat cat = other as Cat;
        if (cat == null) return false; // null or not a cat

if (Name != cat.Name) return false;
        if (!Birthday.Equals(cat.Birthday)) return false;

return true;
    }

public override int GetHashCode()
    {
        unchecked
        {
            int result;
            result = Name.GetHashCode();
            result = 29 * result + Birthday.GetHashCode();
            return result;
        }
    }    
}
```

Why does NHibernate delete then insert composite-elements on select?

Solution

Problem