If Exists Dont Add Data Entity Framework Many-To-Many - entity-framework

Question

I have these two Models the logic is here One Post can have multiple Categories.
public class Post
{
public Post()
{
this.Categories = new HashSet<Category>();
}
public int Id { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public string ShortDescription { get; set; }
public string PostImage { get; set; }
public string Thumbnail { get; set; }
public DateTime CreatedDate { get; set; }
public DateTime? PublishedDate { get; set; }
public string CreatedBy { get; set; }
public virtual ICollection<Category> Categories { get; set; }
}
public class Category
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Post> Posts { get; set; }
}
I have three static categories.
When I am trying to add new post its multiplexing CategoryTable creating new categories with same name ,And Mapping Them in to CategoryPostsTable.
The problem is here i want to map that data with existing categories. I dont want to add new category with same name.
I am using Repository Pattern how should i control that ? Is EF has some solution for that ?

Answer 1

I assume you have code like:
var post = new Post();
post.Categories.Add(cat);
context.Posts.Add(post);
...where cat is a Category object representing an existing category.
The latter Add method (DbSet.Add) doesn't only mark the received entity as Added but all entities in its object graph that are not attached to the context. So cat will also be marked as Added if it wasn't attached yet.
What you can do is
context.Entry(cat).State = System.Data.Entity.EntityState.Unchanged;
Now EF will only create the association to the category, but not insert a new category.