C#•13mo ago

[EF Core 8] Many-to-many relationship with payload

Hi all, I am trying to create a many-to-many relationship as described in https://learn.microsoft.com/en-us/ef/core/modeling/relationships/many-to-many#many-to-many-and-join-table-with-payload My models are the following:

public class Post
{
    public int Id { get; set; }
    public List<Tag> Tags { get; } = new();
    public List<PostTag> PostTags { get; } = new();
}

public class Tag
{
    public int Id { get; set; }
    public List<Post> Posts { get; } = new();
    public List<PostTag> PostTags { get; } = new();
}

public class PostTag
{
    public int PostId { get; set; }
    public int TagId { get; set; }
    public string CustomPayload { get; set; }
}

public class Post
{
    public int Id { get; set; }
    public List<Tag> Tags { get; } = new();
    public List<PostTag> PostTags { get; } = new();
}

public class Tag
{
    public int Id { get; set; }
    public List<Post> Posts { get; } = new();
    public List<PostTag> PostTags { get; } = new();
}

public class PostTag
{
    public int PostId { get; set; }
    public int TagId { get; set; }
    public string CustomPayload { get; set; }
}

MyDbContext:

public class MyDbContext : DbContext
{
    public MyDbContext(DbContextOptions<MyDbContext> options) : base(options) { }
    public virtual DbSet<Post> Posts { get; set; }
    public virtual DbSet<Tag> Tags { get; set; }
}

public class MyDbContext : DbContext
{
    public MyDbContext(DbContextOptions<MyDbContext> options) : base(options) { }
    public virtual DbSet<Post> Posts { get; set; }
    public virtual DbSet<Tag> Tags { get; set; }
}

when I try to create the initial migration I get the following error:

Unable to create a 'DbContext' of type ''. The exception 'The entity type 'PostTag' requires a primary key to be defined. If you intended to use a keyless entity type, call 'HasNoKey' in 'OnModelCreating'

Unable to create a 'DbContext' of type ''. The exception 'The entity type 'PostTag' requires a primary key to be defined. If you intended to use a keyless entity type, call 'HasNoKey' in 'OnModelCreating'

When adding a primary key on the PostTag model:

Unable to create a 'DbContext' of type ''. The exception 'Cannot use table 'PostTag' for entity type 'PostTag' since it is being used for entity type 'PostTag (Dictionary<string, object>)' and potentially other entity types, but there is no linking relationship. Add a foreign key to 'PostTag' on the primary key properties and pointing to the primary key on another entity type mapped to 'PostTag'.

Unable to create a 'DbContext' of type ''. The exception 'Cannot use table 'PostTag' for entity type 'PostTag' since it is being used for entity type 'PostTag (Dictionary<string, object>)' and potentially other entity types, but there is no linking relationship. Add a foreign key to 'PostTag' on the primary key properties and pointing to the primary key on another entity type mapped to 'PostTag'.

I have a few questions: 1) What am I missing and cannot make this relationship function? 2) Even if somehow the tables are produced from the migration, will the PostTag table be able to return the CustomPayload and how? 3) Is this supposed to be the correct way to supply the join table with a user defined payload ?

Many-to-many relationships - EF Core

How to configure many-to-many relationships between entity types when using Entity Framework Core

2 Replies

Angius•13mo ago

You might have to configure it I believe the errors are due to how EF now treats keyless entities as owned types So might need explicit relationship configured there Or at least a composite key on the join entity FWIW I have all my many-to-many with explicit join entities configured explicitly

parko3•12mo ago

Add this to your DbContext: protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder.Entity<Post>() .HasMany(e => e.Tags) .WithMany(e => e.Posts) .UsingEntity<PostTag>( l => l.HasOne(e => e.Tag).WithMany(e => PostTags).HasForeignKey(e => e.TagId), l => l.HasOne(e => e.Post).WithMany(e => PostTags).HasForeignKey(e => e.PostId); }

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[EF Core 8] Many-to-many relationship with payload