How to code asp.net Core 5 new Many to many relationship

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Basquiat

Participant

1852 Points

915 Posts

How to code asp.net Core 5 new Many to many relationship

Jan 31, 2021 12:28 PM|Basquiat|LINK

Hi, asp.net core 5 now allows for many to many relationships.

I understand the model configuration but need help on how to actually make us of the relationship in the controller and views.

public class Post
    {
        public int ID { get; set; }
        public string PostName { get; set; }

        public ICollection<Tag> Tags { get; set; }
}

public class Tag
{ 
public int ID { get; set; } 
public string TagName { get; set; }

public ICollection<Post> Posts { get; set; } 
}

And I can see that EF created a PostTag table in the database.
I ran the scaffolding and created some Posts and created some Tags.

But now I am stuck at added a Tag to a Post, where this is straight forward (and the setup done by the scaffolding) for one to many relationships.

Let's say I want to Edit a Post and from there add some Tags to it, how would one do that?
With a one to many relationship, you would have added TagID as a property to the Post model.

 [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Edit(int id, [Bind("ID,PostName")] Post post)
        {
            if (id != post.ID)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(post);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!PostExists(post.ID))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction(nameof(Index));
            }
            return View(post);
        }

I have no idea what to add to the View to select the Tags nor what I should add to the Controller to Edit the post and link some Tags.

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mgebhard

All-Star

53031 Points

23611 Posts

Re: How to code asp.net Core 5 new Many to many relationship

Jan 31, 2021 12:46 PM|mgebhard|LINK

Basquiat
Let's say I want to Edit a Post and from there add some Tags to it, how would one do that?
With a one to many relationship, you would have added TagID as a property to the Post model.

I strongly recommend going through the Getting Started tutorials found on this site. The tutorials covers reading and updating related records.

https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/?view=aspnetcore-5.0

There are several MVC constructs that you need learn and the tutorial does a very good job of explaining the programming patterns.

Reply

Basquiat

Participant

1852 Points

915 Posts

Re: How to code asp.net Core 5 new Many to many relationship

Jan 31, 2021 02:07 PM|Basquiat|LINK

Thanks for the reply.

I guess my questions are:

1) Does asp.net Core 5 now support many to many relationships?

2) Does that mean you no longer have to create a 3rd entity, like below, to support the relationship?

public class TagAssignment
{ 
public int ID { get; set; } 
public int PostID { get; set; } 
public string TagID { get; set; }

public Post post { get; set; } 
public Tag tag { get; set; } 
}

3) It appears EF now creates PostTag table to manage the relationship.

So what I am confused by (not having done programming for a couple of years), is how you read and write to this PostTag table.
I am looking at the tutorial you referred to, but that still appears to have created a 3rd Entity "CourseAssignment" to manage the many to many relationship between "Instructors" and "Courses"