EF Core Advanced Topics Cascade Delete

Cascade delete allows the deletion of a row to trigger the deletion of related rows automatically.

• EF Core covers a closely related concept and implements several different delete behaviors and allows for the configuration of the delete behaviors of individual relationships.
• In Entity Framework Core, the OnDelete Fluent API method is used to specify the delete behavior for a dependent entity when the principal is deleted.

The OnDelete method takes a DeleteBehavior enum as a parameter.

• Cascade: Child/dependent entity should be deleted.
• Restrict: Dependents are unaffected.
• SetNull: The foreign key values in dependent rows should update to NULL.

Setting a foreign key value to null is not valid if the foreign key is not nullable. The following example set a foreign key field to null when the principal is deleted.

using (var context = new EntityContext())
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Book>()
            .HasOne(b => b.Author)
            .WithMany(a => a.Books)
            .OnDelete(DeleteBehavior.SetNull);
    }
}

The following example configures the relationship as required and specifies that dependant rows are deleted when the principal is deleted.

using (var context = new EntityContext())
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Book>()
            .HasOne(b => b.Author)
            .WithMany(a => a.Books)
            .IsRequired()
            .OnDelete(DeleteBehavior.Cascade);
    }
}

Delete Behaviors

There are four delete behaviors.

Optional Relationships (nullable foreign key)

It is possible to save a null foreign key value, which results in the following effects.

Required Relationships (non-nullable foreign key)

It is not possible to save a null foreign key value, which results in the following effects.

Usage

Cascade

• The Cascade option is used when you have entities that cannot exist without a parent, and you want EF to take care of deleting the children automatically.
• It is the default option for entities that cannot exist without a parent usually make use of required relationships.

ClientSetNull

• The ClientSetNull is used when you have entities that may or may not have a parent, and you want EF to take care of nulling out the foreign key for you.
• It is the default option for entities that can exist without a parent usually make use of optional relationships.

SetNull

• The SetNull option is used when you want the database to also try to propagate null values to child foreign keys even when the child entity is not loaded.
• However, note that the database must support this, and configuring the database like this can result in other restrictions, which in practice often makes this option impractical. This is why SetNull is not the default.

Restrict

• The Restrict option is used when you don't want EF Core to ever delete an entity automatically or null out the foreign key automatically.
• It requires that your code keep child entities and their foreign key values in sync manually, otherwise constraint exceptions will be thrown.