EF Core Cascading Referential Integrity with DeleteBehavior.Restrict does not work well - ef-core-3.1

Question

I have one sql server database created with code first. There are two tables that have a one to many relationship. The database works and is created well.
In sql server if I try to delete one of the classification records, I get an error (referencial integrity restriction). This is how I want it to work. But in ef core, if I delete one classification dbset.Remove(classification), the classification is deleted and the classification in the customer is set to null.
I think this is how it should work for DeleteBehavior.ClientSetNull.
There is a note "Changes in EF Core 2.0" in https://learn.microsoft.com/en-us/ef/core/saving/cascade-delete that explains the DeleteBehavior function.
I have the next records:
Classification:
Id Name
1 General
2 Others
Customers:
Id Name IdClassification
1 Customer A 1
2 Customer B 2
3 Customer C <null>
public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
...
public int? IdClassification { get; set; }
public Classification Classification { get; set; }
}
public class Classification
{
public int Id { get; set; }
public string Name { get; set; }
...
public ICollection<Customer> Customers { get; set; }
}
public class Context : DbContext
{
public virtual DbSet<Classification> Classifications { get; set; }
public virtual DbSet<Customer> Customers { get; set; }
...
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Classification>(
entity =>
{
entity.HasKey(e => e.Id);
});
modelBuilder.Entity<Customer>(
entity =>
{
entity.HasKey(e => e.Id);
entity.HasIndex(e => e.IdClassification);
...
// Claves foráneas
entity.HasOne(c => c.Classification)
.WithMany(x => x.Customers)
.HasForeignKey(x => x.IdClassification)
.OnDelete(DeleteBehavior.Restrict)
.HasConstraintName("FK_Customer_Classification");
});
}
}
Is there a way to prevent deletion of classification records in ef core? (I don't want to check if there is any customer record that is linked to the classification because I have to use the classification with more tables).
Thanks in advance.

Answer 1

EF Core 3.0 added several new values to the DeleteBehavior enum - ClientCascade, NoAction, ClientNoAction. Unfortunately the documentation is not updated (except for enum values in API reference), and only the ClientNoAction is mentioned in the 3.0 Breaking Changes - DeleteBehavior.Restrict has cleaner semantics:
Old behavior
Before 3.0, DeleteBehavior.Restrict created foreign keys in the database with Restrict semantics, but also changed internal fixup in a non-obvious way.
New behavior
Starting with 3.0, DeleteBehavior.Restrict ensures that foreign keys are created with Restrict semantics--that is, no cascades; throw on constraint violation--without also impacting EF internal fixup.
Why
This change was made to improve the experience for using DeleteBehavior in an intuitive manner, without unexpected side-effects.
Mitigations
The previous behavior can be restored by using DeleteBehavior.ClientNoAction.
More info is contained in the associated tracking issue - 12661: Update DeleteBehavior to be more consistent and understandable
Honestly even after reading all that, I don't find it cleaner, but even more confusing. Restrict seems to be obsoleted and replaced with NoAction, which regardless of what have been said actually does set loaded related entities navigation property/FK to null, thus causing SET NULL database behavior as you already experienced.
After trying all of them, the only option which does what you expect is the aforementioned ClientNoAction:
Note: it is unusual to use this value. Consider using ClientSetNull instead to match the behavior of EF6 with cascading deletes disabled.
For entities being tracked by the DbContext, the values of foreign key properties in dependent entities are not changed when the related principal entity is deleted. This can result in an inconsistent graph of entities where the values of foreign key properties do not match the relationships in the graph.
If the database has been created from the model using Entity Framework Migrations or the EnsureCreated() method, then the behavior in the database is to generate an error if a foreign key constraint is violated.
regardless of their note at the beginning.
With all that being said, simply replace Restrict with ClientNoAction and the issue will be solve. No database migration is needed because this change affects only the client behavior.

Answer 2

Well, the classification entity needs correct initialization, suppose to delete restriction rule.
modelBuilder.Entity<Classification>()
.HasKey(e => e.Id)
.HasMany(e => e.Customers)
.WithOne(e => e.Classification)
.OnDelete(DeleteBehavior.Restrict)
.IsRequired(true);
Hope this helps.