Lost every ID from foreign tables and replaced with NULLs

Question

I build my RESTful webservice and after recreating model (replacing enums with strings) and database I have NULLs where there should be Guids of another rows in tables.

this is what I see in SQL Explorer in VS

MIGRATION METHODS:

public override void Up()
    {
        CreateTable(
            "dbo.Artists",
            c => new
                {
                    Id = c.Guid(nullable: false),
                    FirstLastName = c.String(),
                })
            .PrimaryKey(t => t.Id);

        CreateTable(
            "dbo.Pictures",
            c => new
                {
                    Id = c.Guid(nullable: false),
                    Title = c.String(),
                    Path = c.String(),
                })
            .PrimaryKey(t => t.Id);

        CreateTable(
            "dbo.PieceOfArts",
            c => new
                {
                    Id = c.Guid(nullable: false),
                    Title = c.String(),
                    Description = c.String(),
                    DateOfCreation = c.Int(nullable: false),
                    Period = c.String(),
                    Techinque = c.String(),
                    Artist_Id = c.Guid(),
                    Picture_Id = c.Guid(),
                })
            .PrimaryKey(t => t.Id)
            .ForeignKey("dbo.Artists", t => t.Artist_Id)
            .ForeignKey("dbo.Pictures", t => t.Picture_Id)
            .Index(t => t.Artist_Id)
            .Index(t => t.Picture_Id);

    }

    public override void Down()
    {
        DropForeignKey("dbo.PieceOfArts", "Picture_Id", "dbo.Pictures");
        DropForeignKey("dbo.PieceOfArts", "Artist_Id", "dbo.Artists");
        DropIndex("dbo.PieceOfArts", new[] { "Picture_Id" });
        DropIndex("dbo.PieceOfArts", new[] { "Artist_Id" });
        DropTable("dbo.PieceOfArts");
        DropTable("dbo.Pictures");
        DropTable("dbo.Artists");
    }
}

SAMPLE SEED OF THIS COMPLEXED CLASS (PieceOfArt) :

context.PieceOfArts.AddOrUpdate(x => x.Title,
            new PieceOfArt()
            {
                Id = Guid.NewGuid(),
                Title = "Syrenka warszawska",
                Techinque = "Fresk",
                Artist = context.Artists.FirstOrDefault(x => x.FirstLastName.Contains("Picasso")),
                Description = "123",
                Period = "Pierwsza awangarda",
                Picture = context.Pictures.FirstOrDefault(x => x.Title == "Syrenka warszawska"),
                DateOfCreation = 1946,
            },