C# 如何使用主键-实体框架获取列的特定值？

我有不同的桌子。 例如，表1包含人员的姓名 表2包含国家名称和个人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();

使用

---

您应该考虑在person对象上创建一个映射，以便EF可以自动为您映射关系。然后，您可以使用include在一个查询中选择延迟加载或快速加载国家/地区。

方法是为Person类指定一个国家/地区属性并

include（）

ing该导航属性。你试过什么？

//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

//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