Btukfyl

I am learning EF Core. Say I have a many to many relationship with a junction table like this:

public class PostTag

{

    public int PostId { get; set; }

    public Post Post { get; set; }



    public int TagId { get; set; }

    public Tag Tag { get; set; }

}

Say I wanted to perform a database insert.

var post = new Post { Id=Guid.NewGuid(), Description="Test Post" };

var tag = new Tag { Id=Guid.NewGuid(), Description="Test Text"  };

var PostTag = new PostTag { Post=post; Tag=tag; PostId=Post.Id, TagId=Tag.Id};

context.Add(PostTag);

This one insert appears to:

1) Insert a post.

2) Insert a Tag

3) Insert a PostTag

Q1) Why does PostTag contain PostId and TagId? This information is contained in Post and Tag (also two fields in PostTag). It appears that these fields are duplicated.

Q2) Does adding a PostTag also add the Post and the Tag?

Why does PostTag contain PostId and TagId? This information is contained in Post and Tag (also two fields in PostTag). It appears that these fields are duplicated.

As you know, the PostTag.PostId is a foreign key that references the Post.Id, and the PostTag.TagId is a foreign key which references the Tag.Id. The PostTag.PostId called Foreign Key Property and PostTag.Post is Navigation Property.

Actually, if you don't have a foreign key property for navigation property explicitly, the EF Core will also work as expected. In that case, two shadow foreign key properties of PostId and TagId will be introduced for you automatically. It is called Shadow Properties.

Shadow properties can be created by convention when a relationship is discovered but no foreign key property is found in the dependent entity class. In this case, a shadow foreign key property will be introduced. The shadow foreign key property will be named <navigation property name><principal key property name> (the navigation on the dependent entity, which points to the principal entity, is used for the naming). If the principal key property name includes the name of the navigation property, then the name will just be <principal key property name>. If there is no navigation property on the dependent entity, then the principal type name is used in its place.

1. 
   

   Although these foreign key can be inferred by the PostTag.Post.Id and PostTag.Tag.Id respectively, they're not duplicated.

   
   
   

   Let's say we have an existing database where the foreign keys are named as fk_post_id and fk_tag_id, and you want to rename the foreign key property PostId to MyPostId, we cannot omit the PostId and TagId properties :

   
   
   

   public class PostTag
   
   {
   
       [Column("fk_post_id")]
   
       public Guid MyPostId { get; set; }
   
       [ForeignKey("MyPostId")]
   
       public Post Post { get; set; }
   
   
   
   
   
       [Column("fk_tag_id")]
   
       public Guid TagId { get; set; }
   
       public Tag Tag { get; set; }
   
   }
   
   

   
   



2. It is recommended to have a foreign key property defined in the dependent entity class for the navigation property

Does adding a PostTag also add the Post and the Tag?

Your code inserts a post and tag automatically for you because it knows that you need to create a brand new post and a brand new tag. But that's not always that case. It depends on the tracking state and the entity itself.

If you execute the following code :

    var tagTracked = this._dbContext.Tag.FirstOrDefault();

    var post = new Post { Id=Guid.NewGuid(), Description="Test Post3" };

    var PostTag = new PostTag { 

        Post=post, 

        Tag=tagTracked, 

        MyPostId= post.Id, 

        TagId= tagTracked.Id,

    };

    this._dbContext.Add(PostTag);

    this._dbContext.SaveChanges();

The tagTracked will not be inserted . If you want to insert a new tag record , you need to make the tag untracked :

    var tagTracked = this._dbContext.Tag.AsNoTracking().FirstOrDefault();

    tagTracked.Id = Guid.NewGuid();

    var post = new Post { Id=Guid.NewGuid(), Description="Test Post3" };

    var PostTag = new PostTag { 

        Post=post, 

        Tag=tagTracked, 

        MyPostId= post.Id, 

        TagId= tagTracked.Id,

    };

    this._dbContext.Add(PostTag);

    this._dbContext.SaveChanges();

Again, it will insert a new tag record in the database. For more information, refer docs here