fluent api

modelBuilder.Entity<UserRoles>
    .HasRequired(d => d.User)
    .WithMany(p => p.UserRoles)
    .HasForeignKey(d => d.UserId)
    .WillCascadeOnDelete();

modelBuilder.Entity<Role>
    .HasMany(p => p.UserRoles)
    .WithRequired(d => d.Role)
    .HasForeignKey(d => d.RoleId)
    .WillCascadeOnDelete();