lycian Posts - Answer Overflow

lycian

•Created by lycian on 4/14/2024 in #help

Many to Many with Entity Framework

I am an absolute newbie to EF, so keep that in mind. I would have thought that if I had the following

class A
{
    public int Id { get; set; }
    public ICollection<B> B { get; set; } = [];  // Edit: forgot the property name
}

class B
{
  public int Id { get; set; }
}

class A
{
    public int Id { get; set; }
    public ICollection<B> B { get; set; } = [];  // Edit: forgot the property name
}

class B
{
  public int Id { get; set; }
}

and the following model creation

public DbSet<A> A {get; set;}
public DbSet<B> B {get; set;}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    // B doesn't have a loop back 
    // to A, but it's still a many-to-many relationship
    modelBuilder.Entity<A>()
        .HasMany<B>()
        .WithMany();
}

public DbSet<A> A {get; set;}
public DbSet<B> B {get; set;}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    // B doesn't have a loop back 
    // to A, but it's still a many-to-many relationship
    modelBuilder.Entity<A>()
        .HasMany<B>()
        .WithMany();
}

then if I needed to insert a new A I wouldn't have to do anything other than

await context.AddAsync(newA);
await context.SaveChangesAsync();

await context.AddAsync(newA);
await context.SaveChangesAsync();

but I'm getting an error with

Microsoft.Data.SqlClient.SqlException (0x80131904): Cannot insert explicit value for identity column in table 'B' when IDENTITY_INSERT is set to OFF.

Microsoft.Data.SqlClient.SqlException (0x80131904): Cannot insert explicit value for identity column in table 'B' when IDENTITY_INSERT is set to OFF.

I tried reading through the many-to-many docs but didn't find anything in particular: https://learn.microsoft.com/en-us/ef/core/modeling/relationships/many-to-many I would have thought if the id already existed, then it would be inserting a relationship with the implicitly created mapping table. Is there a different way I should be adding A?

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