在实体框架中使用父子FK

Question

There are some situations using Foreign Key in a table that FK has Parent or Child relations with another table as shown below:

Staff:

Id   |  Name  |  CityId |
-------------------------
1001 |  John  |  1      |
1002 |  Mary  |  2      |
1003 |  Bill  |  3      |
1004 |  Jose  |  4      |
1005 |  Anna  |  5      |

City:

Id  |  Name   |  CountryId |
----------------------------
1   |  NY     |  101       |
2   |  Paris  |  102       |
3   |  London |  103       |
4   |  Rome   |  104       |
5   |  Tokyo  |  105       |

Country:

Id  |  Name   | 
---------------
101 |  USA    |
102 |  France |
103 |  UK     |
104 |  Italy  |
105 |  Japan  |

My question is that: Should we use only CityId as FK in the Staff entity, or is it better to include CityId and its parent CountryId in the Staff entity? Of course it seems to be redundant to include a FK and its parent as FK, but I wanted to be clarified if there is some situations or requirements when using such a king of relations with cascade? Which one should be used?

Answer 1

I would avoid breaking normalization because there is no simple way to enforce that a staff's Country reference and it's City's Country reference are guraranteed to be in sync. For instance, a Staff might have a City set to "Toronto" with a Country set to "Canada", but then somewhere the City is updated to "Boston", but Staff.Country is not updated. Staff says country is "Canada" while it's City says country is "USA". Who is trusted as the source of truth?

When it comes to displaying information about staff, use View Models to flatten out relevant details. If you want to display staff details with a Country name and nothing else about the city, the view model can expose that based on data selected from the entities. For instance:

var staffViewModels = context.Staff
    .Select(x => new StaffViewModel
    {
        StaffId = x.Id,
        Name = x.Name,
        Country = x.City.Country.Name
    }).ToList();

Even if the structure introduced an Address for staff which consisted of a City which related to a country:

var staffViewModels = context.Staff
    .Select(x => new StaffViewModel
    {
        StaffId = x.Id,
        Name = x.Name,
        Country = x.Address.City.Country.Name
    }).ToList();

The entity can remain normalized and generate the SQL to efficiently access the relevant data.