Moq: How to retrieve ID of mocked object

I have successfully created a mocked Person object and I am now trying to use my AddChild() method to add a child to the object via its' Id. My test looks like this:

public class PersonManagerMockTest
{
    private static Guid personGuid;

    [ClassInitialize]
    public static void Init(TestContext test)
    {
        personGuid = Guid.NewGuid();
    }

    [TestMethod]
    public void AddNewPerson()
    {
        //Arrange
        var mockDbSet = new Mock<DbSet<Person>>();
        mockDbSet.Setup(x => x.Add(It.IsAny<Person>()))
            .Returns<Person>(p => p);

        var mockContext = new Mock<IHiveTiesContext>();
        mockContext.Setup(x => x.People)
            .Returns(mockDbSet.Object);
        var manager = new Manager(mockContext.Object);

        //Assert
        var _person = manager.CreatePerson("Winston", "Gabriel", DateTime.Now, 'M', personGuid);

        var fatherid = mockDbSet.Object.SingleOrDefault(x => x.RowId == personGuid).Id;
        manager.AddChild(new Person
        {
            FirstName = "Aaron",
            LastName = "Gabriel",
            DOB = new DateTime(1991, 01, 16),
            FavoriteColor = "Red",
            FatherId = fatherid,
        });

        //Act
        mockDbSet.Verify(x => x.Add(It.IsAny<Person>()), Times.Once);
        mockContext.Verify(x => x.SaveChanges(), Times.Once);
    }
}

The mocked Person object is successfully created, but the problem comes from this statement:

var fatherid = mockDbSet.Object.SingleOrDefault(x => x.RowId == personGuid).Id;

I am not sure how to retrieve the Id from the mocked Person object, and as I thought this statement returned 0 for fatherid meaning that it is null. This is my first time running mocks, so I am still learning. Any suggestions on how to retrieve the Id from this mocked Person object? I will appreciate all suggestions. Thank you.