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, you need to pass a new list of Id
for the Tag
entities from the client-side.
Typically, you'd want to define a DTO like -
public class PostUpdateDTO
{
public int Id { get; set; }
public string Title { get; set; }
public List<int> Tags { get; set; }
}
For the updating 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 tags in the database first
var tags = await _DbCtx.Tags.ToListAsync();
foreach (var id in dto.Tags)
{
post.Tags.Add(tags.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 existing Tag
. If you don't like that, 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 the Id :
Neither of these two entities contains the kind of Id (a foreign-key) you are referring to, 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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