2 Foreign Keys as Primary Key using EF Core 2.0 Code First

Question

I have two tables: Comment and Like.

public class Comment {
  CommentID { get; set; }
  ....
}

public class Like {
  CommentID { get; set; }
  UserID { get; set; }
}

Using entity framework core 2.0 code first, I want to define my "Like" model as only having the two fields which reference other primary keys (as foreign keys) but also I want the combination of the values to be the Primary Key of the table. Any help would be appreciated!

See [Keys (primary)](https://learn.microsoft.com/en-us/ef/core/modeling/keys), specifically the part starting with *"You can also use the Fluent API to configure multiple properties to be the key of an entity (known as a composite key)"* — Ivan Stoev, May 17 '18 at 19:12

JohnyL · Accepted Answer · 2018-05-17T19:39:42.573

So, here's what's going on here:

1) There are two classes: Comment and User.

2) The third class, Like, holds references (navigation properties) to both two those classes which correspond to foreign keys in database: UserId and CommentId. I explicitly used ForeignKey attribute, so that it would be clear to you which properties EF will use as foreign keys. In this particular case you could omit this attribute since EF will figure it out automatically (since names match in both classes). Note that it's not mandatory to have foreign keys, but they have advantages.

3) The UserId and CommentId comprise composite key. The Column attribute configures the order of columns in database (so called ordinals). This is important for EF.

4) The User and Comment classes also have navigation properties (since it's one side of one-to-many relations): Likes.

5) Finally, always use Table attribute to avoid problems with pluralizations because there's no way to turn it off.

[Table("Comment")]
public class Comment
{
    public int CommentID { get; set; }
    public List<Like> Likes { get; set; }
}

[Table("User")]
public class User
{
    public int UserId { get; set; }
    public List<Like> Likes { get; set; }
}

[Table("Like")]
public class Like
{
    [Key]
    [Column(Order = 1)]
    public int CommentID { get; set; }
    [Key]
    [Column(Order = 2)]
    public int UserID { get; set; }

    [ForeignKey("CommentId")]
    public Comment Comment { get; set; }
    [ForeignKey("UserId")]
    public User User { get; set; }
}

UPDATE

Setting composite key in EF Core

The Key (and Column) attributes, used to designate composite primary key, actually, don't work in EF Core - they work in EF6. To configure composite key in EF Core, you need to use Fluent Configuration.

You have two options to do it.

OPTION 1. Make all the configuration in OnModelCreatingMethod:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Like>().HasKey(l => new { l.CommentID, l.UserID });
}

OPTION 2. Move all the configuration into separate class and apply it in OnModelCreating:

1) Create separate class for configuration

class LikeConfiguration : IEntityTypeConfiguration<Like>
{
    public void Configure(EntityTypeBuilder<Like> builder)
    {
        builder.HasKey(l => new { l.CommentID, l.UserID });
    }
}

2) Apply configuration:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.ApplyConfiguration(new LikeConfiguration());
}

Choose any option you like.

As you see, to configure composite key in Fluent Configuration, you need to use anonymous type. And again, the order of properties matters.

could you please help answer this question: https://stackoverflow.com/questions/56584027/entity-framework-insert-conflicts-with-another-foreign-key — FrankDupree, Jun 16 '19 at 10:24

2 Foreign Keys as Primary Key using EF Core 2.0 Code First

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