Connecting Id's in many to many table - Entity Framework - entity-framework

Question

I already have many Tags and I already have many Posts. I just want to post 2 Id's and insert those 2 Id's in my many to many table so the 2 records are linked.
I'm using Entity Framework and Fluent Api.
Table 1 called Tags,
Table 2 called Posts,
Table 3 called TagsPosts
My TagsPosts table has the following:
Tag_Id
Post_Id
I just want to add a new record with these 2 Ids like so:
var entry = new TagPost()
{
Tag_Id = tagId,
Post_Id = postId
};
ApplicationDbContext.TagsPosts.Add(entry);
In my context I have:
public class ApplicationDbContext
{
public DbSet<Tag> Tags{ get; set; }
public DbSet<Post> Posts{ get; set; }
public DbSet<TagPost> TagsPosts { get; set; }
}
My fluent API relationship:
ToTable("Tags");
HasKey(t => t.Id);
HasMany(t => t.Posts).WithMany(p => p.Tags);
The issue is i'm getting the error when I try and add the migration using code first:
EntityType 'TagPost' has no key defined. Define the key for this EntityType.
TagsPosts: EntityType: EntitySet 'TagsPosts' is based on type 'TagPost' that has no keys defined.
This is what TagPost looks like:
public class TagPost
{
public int Tag_Id { get; set; }
public int Post_Id { get; set; }
}
What am I doing wrong?

Answer 1

You don't have to include your TagPost class if you map this M2M relationship by convention.
modelBuilder.Entity<Tag>()
.ToTable("Tags")
.HasKey(t => t.Id)
.HasMany(t => t.Posts)
.WithMany(p => p.Tags)
.Map(cs =>
{
cs.MapLeftKey("Tag_Id");
cs.MapRightKey("Post_Id");
cs.ToTable("TagPost");
});
Read this for further info: http://www.entityframeworktutorial.net/code-first/configure-many-to-many-relationship-in-code-first.aspx
EDIT:
//get post object from context
var post = context.Posts.Find(postId);
//get tag object from context
var tag = context.Tags.Find(tagId);
//associate objects
post.Tags.Add(tag);
//commit to db
context.SaveChanges();