如何使用主鍵-實體框架獲取列的特定值？

Question

我有不同的桌子。 例如，表 1 包含 Persons 的名稱 表 2 包含 Country name 和 Person Id

Table 1:
PersonId     PersonName
1            John
2            Smith
3            Kelly


Table 2:
LocationId   Continent      Country     PersonId
1            Asia           Japan       1
2            Asia           China       2

// There is C# method where PersonId should be passed.. suppose id=2 is passed

var person = await _repository.Person.FirstOrDefaultAsync(x => x.PersonId == id); //This find Smith from DB
var location = await _repository.Location.FirstOrDefaultAsync(x => x.LocationId == person.PersonId);   
// location variable will get all values of second row from Table 2 i.e LocationID=2, Continent=Asia, Country=China, PersonId=2
// but i just need value of Country column i.e China of PersonId=2

// Though I can get CountryName from this -- have to write below extra code for that
var country = location.Country;   // this will get Country = China

How can I achieve this in a single line?

// something like
var location = await _repository.Location.FirstOrDefaultAsync(x => x.LocationId == person.PersonId).Country();

Answer 1

使用Select

var country = await _repository.Location.Where(x => x.LocationId == person.PersonId).Select(l => l.Country).FirstOrDefault();

Answer 2

您應該查看在人 object 上創建導航屬性，以便 EF 可以自動為您 map 關系。 然后，您可以選擇延遲加載或在一個包含包含的查詢中急切加載國家/地區。

Answer 3

//First Method :

Entity await _repository = new Entity() // object of your entity.
var Result = (from a in await _repository.Person.Where(a=>a.PersonId == id)
              from b in await _repository.Location.Where(b=>b.PersonId=a.PersonId)
              select new 
              {
                 a.PersonId,a.PersonName,b.Country
              }).SingleOrDefault();
Result.Country; // Here you can get country,also get other values PersonName,PersonId

//Second Method :

Entity await _repository = new Entity() // object of your entity.
var Result = (from a in await _repository.Location.Where(a=>a.PersonId == id)select 
              a.Country).SingleOrDefault(); //other wise you can select 'a'.  
Result.Country; // Here you can get country