C# Entity Framework: 将 select 结果映射到对象列表

Question

假设我有一个学生对象列表，我想找到他们的学费。他们的学费在数据库中，关键是他们的学生 ID。之后我想将该学费映射到学生对象中。所以我有

Class Student{
    int Id;
    string Name;
    double GPA;
    double Tuition; 
}

    List<Double> GetTuitionsById(List<Student> students)
    {
        var ids = students.Select(x => x.Id).ToList();
        var query = from dbStudent in _context.StudentsDB
                    where ids.Contains(dbStudent.Id)
                    select dbStudent.Tuition;

        var tuitions = query.ToList();

        return tuitions;
    }

然后我希望遍历 tuitions 并将其映射到学生列表中，但是 select 查询并没有 return 相同顺序的价格。那么如何将学费从数据库映射到学生对象呢？

Answer 1

尝试。

 List<Double> GetTuitionsById(List<Student> students)
    {
     List<Double> objRes=new List<Double>();
        var ids = students.Select(x => x.Id).ToList();
        var query = (from dbStudent in _context.StudentsDB
                    where ids.Contains(dbStudent.Id)
                    select dbStudent.Tuition).ToLIst();
         foreach(var x in query )
         {
          objRes.Add(Double.Parse(x));
         }

        return objRes;
    }

Answer 2

当您在数据库查询中没有排序时，不保证来自数据库的结果按任何特定顺序排列。要 return 按顺序传递的结果，您需要从数据库中 return Id 以便数据可以映射到初始列表，然后手动创建列表：

List<Double> GetTuitionsById(List<Student> students)
{
    var ids = students.Select(x => x.Id).ToList();
    // return Id and tuition
    var query = from dbStudent in _context.StudentsDB
                where ids.Contains(dbStudent.Id)
                select new { dbStudent.Id, dbStudent.Tuition};  

    var tuitions = query.ToList();

    var results = List<double>();
    foreach(var student in students)
    {
        // below is assumed that every student has tuition returned from the database
        var foundTuition = tuitions.First(t => t.Id == student.Id);
        results.Add(foundTuition.Tuition);
    }
    return results;
}

编辑： @pwas 正确地指出使用 Dictionary 会提供更好的性能：

List<Double> GetTuitionsById(List<Student> students)
{
    var ids = students.Select(x => x.Id).ToList();
    // return Id and tuition
    var query = from dbStudent in _context.StudentsDB
                where ids.Contains(dbStudent.Id)
                select new { dbStudent.Id, dbStudent.Tuition};  

    var tuitions = query.AsEnumerable().ToDictionary(s => s.Id, s => x.Tuition);

    var results = List<double>();
    foreach(var student in students)
    {
        // below is assumed that every student has tuition returned from the database
        results.Add(tuitions[student.Id]);
    }
    return results;
}

C# Entity Framework: 将 select 结果映射到对象列表

C# Entity Framework: Mapping select results to list of objects

c#

sql-server

mapping

entity-framework