EF 6.0多对多关系编辑

Question

I'm having a problem with my Entity Framework 6.0 my set up is as follows

public Post
{
    [Key]
    public int Id {get;set;}

    public String Name {get;set;}

    public virtual List<Category> Categories {get;set;}
}

public Category
{
    [Key]
    public int Id {get;set;}

    public string Name {get;set;}

    public virtual List<Post> Posts {get;set;}
}

so the problem arises when I try to modify one of the lists like this

Posts.Categories.Remove(category);
Posts.Categories.Add(newCategory);
entities.SaveChanges();

I get the following exception, and the exception only happens when I try to modify a Post that has already been created and that has categories .

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.

I'm not too sure what to do in this case, should I delete the Post from the Category as well ??? Keep in mind that by removing the Category from the list I just want to remove it from that collection not to remove the whole object from my DB. Any suggestions ?

This is my first post to StackOverflow if somebody needs more info please let me know.

Answer 1

The way I handle these many to many relationships is as follows: (assuming Post is an object from the DB)

            var tmp = Post.Categories.Select(q => q).ToList();
            //delete all links
            foreach (var lab in tmp) {
                Posts.Categories.Remove(lab);
            }
            db.SaveChanges();

            //add new cats
            foreach (var lblgroup in myListofNewCats) {
               Post.Categories.Add(db.Categories.Single(q => q.ID=something);                    
            }

It works out best when you submit your changes after the delete. In case there are no changes, or in case you remove and add the same entity again without submitting in between it might throw some errors.

I'm sure there might be better solutions.

Answer 2

You could define your middle table and then just delete the record in that. That way, you would not be deleting the category itself, which it appears that you are doing now. I suggest that you amend your models as follows:

public Post
{
    [Key]
    public int Id {get;set;}

    public String Name {get;set;}

    //For the many to 1 relationship
    public virtual ICollection <PostCategory> PostCategories{get;set;}

    //You wont need this anymore
    //public virtual List<Category> Categories {get;set;}
}

and...

public Category
{
    [Key]
    public int Id {get;set;}

    public string Name {get;set;}

    //For the many to 1 relationship
    public virtual ICollection <PostCategory> PostCategories{get;set;}

    //You wont need this anymore
    //public virtual List<Post> Posts {get;set;}
}

Now create model for the new table, PostCategory, this will be the middle table. I like to use a single key rather than a double key. You get more flexibility and it is easy to use when using repositories and the out of the box controller delete methods, but you can use a double key if you like - I am not showing that here. In this method, you will need to check for duplicates yourself before adding the record to the database.

public PostCategory 
{
    [Key]
    public int Id {get;set;}  
    public int PostId {get;set;}
    public virtual Post Post {get;set;}

    public int CategoryId {get;set;}
    public virtual Category Category {get;set;}
}

Remember to define "PostCategories" in your dbcontext as well. (I presume you know how to...?)

Now when you want to delete the link between the Post and the Category, just delete the PostCategory record, like this in your controller:

//Find the record where postId is the PostId and the categoryId is the CategoryId
var postRecord = db.PostCategories.FirstOrDefault(x=>x.PostId==postId && x.CategoryId==categoryId);

if(postRecord!=null)
{
   db.PostCategories.Remove(postRecord)
   db.SaveChanges();
}

Adding a record is also easy. I do it like this in the controller...

//First create a record to add
PostCategory pc= new PostCategory()

//wire it up... EF adds the Id fields into the record. If you have a problem
// you can even add those.
pc.Category = category;
pc.Post = post;

//add it
db.PostCategories.Add(pc);
db.SaveChanges();

I like this method because now you can save additional stuff in your PostCategory table, such as date of Post etc. I don't like Many to Many relationships, I believe that they will sooner or later be broken down to one to many and many to one... and later when you have to "fix code" - its a pain to say the least. I hope this helps.