Select LINQ 中来自两个 table 的多个字段 Select 使用 LEFT JOIN 查询

Question

我正在尝试使用 LINQ select 所有 CourseInstances (table) 加入课程 table； CourseInstances table 左边是 courses 右边 table。现在我的 LINQ 查询工作正常，直到我在 select new {..... 我收到错误

中添加字段 '_courses.StudyLevelId'

错误

The name '_courses' does not exist in the current context

我正在使用 LINQPad 进行测试....在底部我提供了 ViewModel 方法代码...

LINQ 查询

from _courseInstances in CourseInstances
join _courses in Courses on _courseInstances.CourseID equals _courses.CourseID into c
from a in c.DefaultIfEmpty()
orderby _courseInstances.CourseCode
select new {_courseInstances.CourseID, _courseInstances.CourseCode, _courseInstances.CourseTitle, _courseInstances.UCASCode, _courseInstances.StartDate, _courses.StudyLevelId }

生成的 SQL 脚本没有 _courses.StudyLevelId

SELECT [t0].[CourseID], [t0].[CourseCode], [t0].[CourseTitle], [t0].[UCASCode], [t0].[StartDate]
FROM [CourseInstances] AS [t0]
LEFT OUTER JOIN [Courses] AS [t1] ON [t0].[CourseID] = ([t1].[CourseID])
ORDER BY [t0].[CourseCode]

我创建了 ModelView 来处理单个查询中的多个 table，但我遇到了同样的错误

C# 中的视图模型

 public class CoursesInstanceStudyLevel_ViewModel
{
    public CourseInstanceModel _CourseInstanceModel { get; set; }
    public CoursesModel _CoursesModel { get; set; }
    public StudyLevelModel  _StudyLevelModel { get; set; }
    public ApplicationURLValidityModel  _ApplicationURLValidityModel { get; set; }
}

C# Linq 查询

var bb2 = 
 (from _courseInstances in _uof.CourseInstances_Repository.GetAll()
  join _courses in _uof.Courses_Repository.GetAll() on _courseInstances.CourseID equals _courses.CourseID into c
  from a in c.DefaultIfEmpty()
  orderby _courseInstances.CourseCode
  select new CoursesInstanceStudyLevel_ViewModel()
        {
          _CourseInstanceModel = _courseInstances,
          _CoursesModel = _courses
        }).ToList();

具有多个连接

我在尝试添加多个连接时遇到以下错误。我相信连接是正确的，与 select{.... 数据为空的位置相同？ UWLApplicationURL = _ApplicationURLValidityRules == 空？ (bool?)null : _ApplicationURLValidityRules.UWLApplicationURL }

 Error :Cannot assign <null> to anonymous type property
 Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'bool'

代码

  from _courseInstances in CourseInstances
  join _coursesList in Courses on _courseInstances.CourseID equals _coursesList.CourseID into c
  from _courses in c.DefaultIfEmpty()
  join _ApplicationURLValidityRulesList in ApplicationURLValidityRules on _courseInstances.CourseInstanceID equals _ApplicationURLValidityRulesList.CourseInstanceID into d
  from _ApplicationURLValidityRules in d.DefaultIfEmpty()
  join _studylevel in StudyLevels on _courses.StudyLevelId equals _studylevel.StudyLevelID
  orderby _courseInstances.CourseCode
  select new {_courseInstances.CourseID, _courseInstances.CourseCode, _courseInstances.CourseTitle, _courseInstances.UCASCode, _courseInstances.StartDate, _courses.StudyLevelId, _studylevel.LevelDescription, _studylevel.SLevelType, UWLApplicationURL = _ApplicationURLValidityRules == null? (bool?)null :  _ApplicationURLValidityRules.UWLApplicationURL }

Answer 1

你必须使用 a 而不是 _courses:

 from _courseInstances in _uof.CourseInstances_Repository.GetAll()
  join _courses in _uof.Courses_Repository.GetAll() on _courseInstances.CourseID equals _courses.CourseID into c
  from a in c.DefaultIfEmpty()
  orderby _courseInstances.CourseCode
  select new CoursesInstanceStudyLevel_ViewModel()
        {
          _CourseInstanceModel = _courseInstances,
          _CoursesModel = a
        }).ToList();

Answer 2

我已经设法解决了第一个问题，感谢 Sandeep 让我知道我应该看哪里。接下来我需要检查多个连接

  var bb2 = (from _courseInstances in _uof.CourseInstances_Repository.GetAll()
                     join _coursesList in _uof.Courses_Repository.GetAll() on _courseInstances.CourseID equals _coursesList.CourseID into c
                     from _courses in c.DefaultIfEmpty()
                     orderby _courseInstances.CourseCode
                     select new CoursesInstanceStudyLevel_ViewModel()
                     {
                         _CourseInstanceModel = _courseInstances,
                         _CoursesModel = _courses 
                     }).ToList();

Select LINQ 中来自两个 table 的多个字段 Select 使用 LEFT JOIN 查询

Select mutiple fields from two table in LINQ Select Query using LEFT JOIN

linq

sql-server

asp.net-mvc

entity-framework

错误

LINQ 查询

生成的 SQL 脚本没有 _courses.StudyLevelId

C# 中的视图模型

C# Linq 查询

具有多个连接

代码