使用多对多映射更新分离的EF对象

Question

I have a repository in EF that needs to Update detached entities but ones which exist in the ObjectStateManager. Imagine you have a session open on a DBContext where you have loaded a particular entity, but then you receive a request to update from a detached version of the object. The only way I have found to do this is to GET the existing object in the state manager, then one by one, update the fields to the fields of the passed-in object. Then set the state of the object state manager version to modified, and save the context.

This works for simple entities that don't contain navigation properties.

I am now trying to do so on an entity that has a many-to-many relationship.

Imagine you have a BlogPost object, and a Hashtag object. This is a many to many relationship. I have defined this and in the database I can see I have three tables, the BlogPost, the HashTag and the mapping table.

What I want to be able to do is edit the blog post on the front end, pass in the updated blog post with it's new list of hashtags that apply to it, and update the database.

The problem is the list of hashtags could be completely unrelated to the old one, so I first have to clear out all the previous mappings, then add the new ones in. If they are the same, this will be a necessary redundancy but the only way to achieve it.

I cannot figure out how to clear out the previous mappings in the many to many relationship though. I have tried

foreach (var tag in dbBlogPost.Hashtags)
    dbItem.Hashtags.Remove(tag);

I then add the new hashtags to the empty collection, then do

Work.Context.Entry(dbItem).State = EntityState.Modified;
Work.Context.Save();

But when I save the repository, I get the following exception

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.

Can anyone suggest what I am doing wrong?

Answer 1

The DbSet<TEntity>.Local represents a local view of all Added, Unchanged, and Modified entities in this set. So you can clear all the collection before make any change.

dbBlogPost.Hashtags.Local.Clear();