How to fix EF Core migration error on join table configuration - entity-framework-core

Question

Using EF core 6.0.1, I'm following the example shown here for configuring the join table for a many-to-many relationship in a migration, but I can't get past this error:
The seed entity for entity type 'Classifier' cannot be added because
no value was provided for the required property 'Id'.
modelBuilder.Entity<Classifier>().HasData(
new Classifier
{
Id = 1,
Name = "Concerto",
}
);
modelBuilder.Entity<Composition>()
.HasData(
new Composition
{
Id = -1,
Name = "First Composition",
CreatorId = -1,
LastModifierId = -1,
CreatedDate = DateTime.Parse("01/01/2019"),
LastModifiedDate = DateTime.Parse("01/01/2019"),
Summary = "Lorem ipsum",
}
);
modelBuilder.Entity<Classifier>()
.HasMany(cl => cl.Compositions)
.WithMany(cm => cm.Classifiers)
.UsingEntity(j => j.ToTable("ClassifierCompositions")
)
.HasData(
new { ClassifiersId = 1, CompositionsId = -1, },
);
I've verified that the names used in the anonymous type used to configure the join match the autogenerated column names from EF. I suspect the error message is a poor representation of the real error, since clearly the Classifier.Id is provided. Why is this error being thrown?
Fwiw, the Composition table exists already as created by a previous migration and this migration is adding the Classifier table and join table.

Answer 1

I suspect the error message is a poor representation of the real error, since clearly the Classifier.Id is provided. Why is this error being thrown?
Actually the error is correct. It's because here
modelBuilder.Entity<Classifier>()
.HasMany(cl => cl.Compositions)
.WithMany(cm => cm.Classifiers)
.UsingEntity(j => j.ToTable("ClassifierCompositions")
) // <-- the problem
.HasData(
new { ClassifiersId = 1, CompositionsId = -1, },
);
you are executing the UsingEntity call, which returns EntityTypeBuilder<Classifier>, so the next HasData call is actually defining more Classifier data entries via anonymous type which of course has no Id property, hence the error message.
The correct action of course is to define the seed data for the join entity, using its entity builder inside the UsingEntity call, i.e.
modelBuilder.Entity<Classifier>()
.HasMany(cl => cl.Compositions)
.WithMany(cm => cm.Classifiers)
.UsingEntity(j => j
.ToTable("ClassifierCompositions")
.HasData(
new { ClassifiersId = 1, CompositionsId = -1, },
)
);