EF Core 2.1 with Seed method and NotMapped attribute

I have a question, why my Strategy property is null, when i getting all DbSet from context? How to [NotMapped] property make visible in my backend?

My class looks like this:

public class Machine
{
    [Key]
    public int Id { get; set; }
    [NotMapped]
    public WorkStrategy Strategy { get; set; }

    public double GetManHours() => Strategy.TimeOfWork(HoursPerDay);
}

WorkStrategy is an abstract class:

public abstract class WorkStrategy
{
    public abstract double TimeOfWork(double hours);
}

public class FarmStrategy : WorkStrategy
{
    public override double TimeOfWork(double hours) => // do things
}

public class CultivationStrategy : WorkStrategy
{
    public override double TimeOfWork(double hours) => //do things
}

Part of my Seed method where i seeding machines looks like this:

//Machines
for(int i = 0; i < countOfMachines; i++)
{
    Machine machine = new Machine { Id = i + 1 };
    machine.Strategy = new FarmStrategy;
    modelBuilder.Entity<Machine>().HasData(machine);
}

But when i call Machines from DB:

var machines = _context.Machines;

The Strategy property is null. Could you tell me, how to attach [NotMapped] property while seeding a db ? Is it possbile?

EDIT

When i want to add WorkStrategy as not "notmapped" i get an error from EF while i adding migration:

The entity type 'WorkStrategy' requires a primary key to be defined

But i dont want to make an table for WorkStrategy.

EDIT My OnModelCreating in context class:

protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Machine>().Ignore(x => x.Strategy);

        builder.Seed();
        base.OnModelCreating(builder);
    }

Its not work as [NotMapped]