I have problems removing all elements from a collection in entity framework using Clear()
Consider the often used example with Blogs and Posts.
public class Blog
{
public int Id {get; set;}
public string Name {get; set;}
public virtual ICollection<Post> Posts { get; set; }
}
public class Post
{
public int Id { get; set; }
// foreign key to Blog:
public int BlogId { get; set; }
public virtual Blog Blog { get; set; }
public string Title { get; set; }
public string Text { get; set; }
}
public class BlogContext : DbContext
{
public DbSet<Blog> Blogs {get; set;}
public DbSet<Post> Posts {get; set;}
}
A Blog has many Posts. A Blog has an ICollection of Posts. There is a straightforward one-to-many relation between Blogs and Posts.
Suppose I want to remove all Posts from a Blog
Of course I could do the following:
Blog myBlog = ...
var postsToRemove = dbContext.Posts.Where(post => post.BlogId == myBlog.Id);
dbContext.RemoveRange(postsToRemove);
dbContext.SaveChanges();
However, the following seems easier:
Blog myBlog = ...
myBlog.Posts.Clear();
dbContext.SaveChanges();
However this leads to an InvalidOperationException:
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.
What is the proper way to clear the collection? Is there a fluent API statement for this?
There is a difference between your two code samples.
Your first code sample
dbContext.RemoveRange(postsToRemove)
removes thePost
records. Therefor, any relationship involving these records are also removed.In your second code sample
myBlog.Posts.Clear()
you are removing the relationship betweenmyBlog
and its correspondingPost
records. The 'real' underlying action is to set the value ofBlogId
of thePost
records tonull
. Unfortunately this is not possible, sinceBlogId
is set to not-nullable. So, in short, the relationship is removed, and no records are actually deleted.Clear
works on the relationship and not on deleting the entity.There is a working (I think more readable) half way solution between the two solutions you wrote.
EDIT
RemoveRange
also removes Posts from theBlog.Posts
collection