EF Core 5.0 - Updating many-to-many entities in ASP.NET Core Web API

Question

With EF Core 5.0 Many-to-many relations are introduced. I'm getting stucked on how to update them through my asp .net api.

For One-to-one and One-to-many relations there is a convention by simply adding the property name followed by ID.

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }

    public BlogImage BlogImage { get; set; }
}

public class BlogImage
{
    public int BlogImageId { get; set; }
    public byte[] Image { get; set; }
    public string Caption { get; set; }

    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

So a propper POST Request could look like

{
  "BlogId": 123,
  "Url": "example.com",
  "BlogImageID": 42
}

but I could not find out if there is a convention or how it look like for Many-to-many relations

public class Post
{
    public int PostId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

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

public class Tag
{
    public string TagId { get; set; }

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

Is there a convention to map the body of a http request to Many-to-many relations using EF 5.0?

atiyar · Accepted Answer · 2021-03-15 11:22:59Z

Consider the following two entities which are in many-to-many relationship -

public class Post
{
    public int Id { get; set; }
    public string Title { get; set; }

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

public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; }

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

When updating the Tags in a Post entity, in the most common scenario, a new list of tag Ids are sent from the client-side, and the request payload will look like -

{
    "id": 123,
    "title": "An Awesome Post",
    "tags": [2, 7, 13]
}

Typically, you'd want to define a DTO to represent this request object, like -

public class PostUpdateDTO
{
    public int Id { get; set; }
    public string Title { get; set; }

    public List<int> Tags { get; set; }
}

Then, for the update operation itself, you can do something like -

[HttpPut]
public async Task Put([FromBody]PostUpdateDTO dto)
{
    // fetch existing Post including related Tags
    var post = await _DbCtx.Posts
        .Include(p => p.Tags)
        .FirstOrDefaultAsync(p => p.Id == dto.Post.Id);

    // remove all Tags from the existing list
    post.Tags.Clear();
    
    // add new Tags to the list whose Ids are sent by the client
    // but to identify them you need the list of all available tags
    var availableTags = await _DbCtx.Tags.ToListAsync();
    foreach (var id in dto.Tags)
    {
        post.Tags.Add(availableTags.First(p => p.Id == id));
    }
    
    // modify properties of Post if you need, like -
    // post.Title = dto.Title;

    await _DbCtx.SaveChangesAsync();
}

As you can see, this requires a trip to the database to fetch a list of all available Tag. If you don't like that and want to skip it, you can try the following approach -

[HttpPut]
public async Task Put([FromBody]PostUpdateDTO dto)
{
    // fetch existing Post including related Tags
    var post = await _DbCtx.Posts
        .Include(p => p.Tags)
        .FirstOrDefaultAsync(p => p.Id == dto.Post.Id);

    // remove Tags which are in the existing Tag list, but not 
    // in the new list sent by the client
    post.Tags.Where(tag => !dto.Tags.Any(id => id == tag.Id))
        .ToList().ForEach(tag => post.Tags.Remove(tag));

    // add Tags which are in the new list sent by the client, but 
    // not in the existing Tag list
    dto.Tags.Where(id => !post.Tags.Any(tag => tag.Id == id))
        .ToList().ForEach(id => post.Tags.Add(new Tag { Id = id }));

    // modify properties of Post if you need, like -
    // post.Title = dto.Title;

    await _DbCtx.SaveChangesAsync();
}

About that - property name followed by ID :
The kind of Id property you are referring to represents a foreign-key. Neither of these two entities contains a foreign-key property, because neither of them depends on the other. A foreign-key implies a parent/child or principal/dependent relationship. But when two entities are in many-to-many relation, they are independent of each other.

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