Asp.Net Core EF Core many to many relation update command - asp.net-core

Question

I followed this link enter link description here to create many to many relationship. But, I do not know how to create and update Tag value to Post Object.
Any help would be appreciated.
Update, related code
class MyContext : DbContext
{
public DbSet<Post> Posts { get; set; }
public DbSet<Tag> Tags { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<PostTag>()
.HasKey(t => new { t.PostId, t.TagId });
modelBuilder.Entity<PostTag>()
.HasOne(pt => pt.Post)
.WithMany(p => p.PostTags)
.HasForeignKey(pt => pt.PostId);
modelBuilder.Entity<PostTag>()
.HasOne(pt => pt.Tag)
.WithMany(t => t.PostTags)
.HasForeignKey(pt => pt.TagId);
}
}
public class Post
{
public int PostId { get; set; }
public string Title { get; set; }
public string Content { get; set; }
public List<PostTag> PostTags { get; set; }
}
public class Tag
{
public string TagId { get; set; }
public List<PostTag> PostTags { get; set; }
}
public class PostTag
{
public int PostId { get; set; }
public Post Post { get; set; }
public string TagId { get; set; }
public Tag Tag { get; set; }
}
Update2: Code for Update record
With below code, it will create records in both three tables.
var p = new Post { Content = "C1" };
var t = new Tag { TagId = "T1" };
var pt = new PostTag { Post = p, Tag = t };
_context.PostTag.Add(pt);
_context.SaveChanges();
But, with below code, it will insert new records in middle table PostTag instead of update the previous records.
var t1 = new Tag { TagId = "T3" };
var t2 = new Tag { TagId = "T4" };
var p =_context.Posts.Find(1);
p.PostTags = new List<PostTag>() {
new PostTag{ Post=p, Tag=t1},
new PostTag{ Post=p, Tag=t2}
};
_context.Posts.Update(p);
_context.SaveChanges();

Answer 1

Let's create some sample data
var p = new Post { ... };
var t = new Tag { ... };
var pt = new PostTag { Post = p, Tag = t };
This is still in-memory so we have to add it to the context, starting with:
db.Posts.Add(p);
db.Tags.Add(t);
Note that at this stage neither p nor t has any knowledge of pt. There are two ways to insert the third entity into your database.
1) The easiest way is to add the DbSet<PostTag> property and then the line
db.PostTags.Add(pt);
2) Without that extra DbSet you will need to ensure the linking from the other 2 sides. You also need to set up the List properties:
p.PostTags = new List<PostTag> { pt };
t.PostTags = new List<PostTag> { pt };
EF will now fill out any missing link fields.