如何加入 List<Class> 和 IEnumerable<Class>？

Question

我有以下课程。

  public class Course 
    {
        public string CourseNumber { get; set; }
        public List<PriceGroup> PriceGroups { get; set; }
    }


   public class PriceGroup
    {
        public int Id { get; set; }
        public string CourseNumber { get; set; }
    }

我有一个从数据库中获取数据的服务。 Courses 是 IEnumerable<Course>，priceGroups 是 IEnumerable<PriceGroup>。正如您在 Course 类中所见，课程具有 List<PriceGroup>。如果它们与变量CourseNumber 匹配，我需要以某种方式将IEnumerable<PriceGroup> 中的每个PriceGroup 添加到IEnumerable<Course> 中每门课程的列表中。所以我最终会得到一个正确的课程，其中包含正确的List<PriceGroup>'s

public async Task<IEnumerable<Course>> GetAllAsync()
        {
            //courses is an IEnumerable<Course>
            var courses = await _courseRepository.GetAllAsync();

            //priceGroups is an IEnumerable<PriceGroup>
            var priceGroups = await _coursePriceGroupRepository.GetAllAsync();     

            courses.GroupJoin(priceGroups, c => c.PriceGroups.Select(i => i.CourseNumber), pg => pg.CourseNumber, (c, pg) => new
            {

            });    
            return await courses;
        }

为此，我尝试进行 GroupJoin，但没有成功。我还没有完成尝试，因为我完全被困在了精神上。也许我正在尝试使用 GroupJoin 做一些不可能的事情。有谁知道实现我需要的方法？

Answer 1

所以你想要所有（或部分）课程，每门课程都有其所有（或部分）PriceGroups。

Courses 和 PriceGroups 之间存在直接的一对多关系：每个 Course 都有零个或多个 PriceGroups，每个 PriceGroup 只属于一个 Course，即外键 CourseNumber 所指的 Course。

你是对的，只要你想要项目及其子项目，你就可以使用 GroupJoin。

public async Task<IEnumerable<Course>> GetAllAsync()
{
    IEnumerable<Course> courses = await _courseRepository.GetAllAsync();
    IEnumerable<PriceGroup> priceGroups = await _coursePriceGroupRepository.GetAllAsync();     

    var queryCoursesWithPriceGroups = courses.GroupJoin( // GroupJoin Courses
        priceGroups,                                     // with PriceGroups
        course => course.CourseNumber,                   // from every Course take primary key
        priceGroup => priceGroup.CourseNumber,           // from every PriceGroup take foreign key
        (course, priceGroupsOfThisCourse) => new        // from every Course with
        {                                               // its priceGroups make one new
             // Select the Course properties you plan to use:
             Id = course.Id,
             Name = course.Name,
             StartDate = course.StartDate,
             ...

             PriceGroups = priceGroupsOfThisCourse.Select(priceGroup => new
             {
                  // Select only the PriceGroup properties that you plan to use
                  Id = priceGroup.Id,
                  Name = priceGroup.Name,
                  ...

                  // not needed, you know the value:
                  // CourseId = priceGroup.CourseId
             })
             .ToList(),
       });
       // Note: this is a query. It is not executed yet!

       return await queryCoursesWithPriceGroups.ToListAsync();

Answer 2

//Assuming courseNumber is unique in Courses table
return Courses.Select( c=> { c.PriceGroup=priceGroup.Where(p=>p.CourseNumber==c.CourseNumber).ToList(); return c; });

Answer 3

我做了一个实际的例子，显然你应该用存储库的方法代替假方法：

class Program
{
    static async void Main(string[] args)
    {
        var courses = GetAllAsync().Result;
    }

    private static async Task<IEnumerable<Course>> GetAllAsync()
    {
        //courses is an IEnumerable<Course>
        var courses = await GetCoursesAsync();

        //priceGroups is an IEnumerable<PriceGroup>
        var priceGroups = await GetPriceGroups();

        foreach (var course in courses)
        {
            foreach (var priceGroup in priceGroups.Where(x => x.CourseNumber == course.CourseNumber))
            {
                course.PriceGroups.Add(priceGroup);
            }
        }

        return courses;
    }

    private static async Task<IEnumerable<Course>> GetCoursesAsync()
    {
        return await Task.FromResult<IEnumerable<Course>>(
            new List<Course>() {
                new Course{
                    CourseNumber = "PIZZA1",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA2",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA3",
                    PriceGroups = new List<PriceGroup>()
                },
            }
        );
    }

    private static async Task<IEnumerable<PriceGroup>> GetPriceGroups()
    {
        return await Task.FromResult<IEnumerable<PriceGroup>>(
            new List<PriceGroup>() {
                new PriceGroup{
                    Id = 1,
                    CourseNumber = "PIZZA1"
                },
                new PriceGroup{
                    Id = 2,
                    CourseNumber = "PIZZA2"
                },
                new PriceGroup{
                    Id = 3,
                    CourseNumber = "PIZZA3"
                }
            }
        );
    }
}

public class Course
{
    public string CourseNumber { get; set; }
    public List<PriceGroup> PriceGroups { get; set; }
}

public class PriceGroup
{
    public int Id { get; set; }
    public string CourseNumber { get; set; }
}

---

从数据库返回数据时应避免使用 IEnumerable，否则可能会导致意外行为。我建议你采用这种方法：

class Program
{
    static void Main(string[] args)
    {
        var courses = GetAllAsync().Result;
    }

    private static async Task<List<Course>> GetAllAsync()
    {
        var courses = await GetCoursesAsync();

        var priceGroups = await GetPriceGroups();

        courses.ForEach(x => { x.PriceGroups.AddRange(priceGroups.Where(y => y.CourseNumber == x.CourseNumber)); });

        return courses;
    }

    private static async Task<List<Course>> GetCoursesAsync()
    {
        return await Task.FromResult(
            new List<Course>() {
                new Course{
                    CourseNumber = "PIZZA1",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA2",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA3",
                    PriceGroups = new List<PriceGroup>()
                },
            }
        );
    }

    private static async Task<List<PriceGroup>> GetPriceGroups()
    {
        return await Task.FromResult(
            new List<PriceGroup>() {
                new PriceGroup{
                    Id = 1,
                    CourseNumber = "PIZZA1"
                },
                new PriceGroup{
                    Id = 2,
                    CourseNumber = "PIZZA2"
                },
                new PriceGroup{
                    Id = 3,
                    CourseNumber = "PIZZA3"
                }
            }
        );
    }
}

public class Course
{
    public string CourseNumber { get; set; }
    public List<PriceGroup> PriceGroups { get; set; }
}

public class PriceGroup
{
    public int Id { get; set; }
    public string CourseNumber { get; set; }
}

---

我也使用 GroupJoin 制作了一个版本：

class Program
{
    static void Main(string[] args)
    {
        var courses = GetAllAsync().Result;
    }

    private static async Task<List<Course>> GetAllAsync()
    {
        var courses = await GetCoursesAsync();

        var priceGroups = await GetPriceGroups();

        var groupedData = courses.GroupJoin(priceGroups,
                course => course.CourseNumber,
                priceGroup => priceGroup.CourseNumber,
                (course, priceGroupsCollection) =>
                new
                {
                    CourseNumber = course.CourseNumber,
                    PriceGroups = priceGroupsCollection.ToList()
                });

        courses.ForEach(x => { x.PriceGroups = groupedData.FirstOrDefault(y => y.CourseNumber == x.CourseNumber)?.PriceGroups ?? new List<PriceGroup>(); });

        return courses;
    }

    private static async Task<List<Course>> GetCoursesAsync()
    {
        return await Task.FromResult(
            new List<Course>() {
                new Course{
                    CourseNumber = "PIZZA1",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA2",
                    PriceGroups = new List<PriceGroup>()
                },
                new Course{
                    CourseNumber = "PIZZA3",
                    PriceGroups = new List<PriceGroup>()
                },
            }
        );
    }

    private static async Task<List<PriceGroup>> GetPriceGroups()
    {
        return await Task.FromResult(
            new List<PriceGroup>() {
                new PriceGroup{
                    Id = 1,
                    CourseNumber = "PIZZA1"
                },
                new PriceGroup{
                    Id = 2,
                    CourseNumber = "PIZZA2"
                },
                new PriceGroup{
                    Id = 3,
                    CourseNumber = "PIZZA3"
                },
                new PriceGroup{
                    Id = 4,
                    CourseNumber = "PIZZA1"
                }
            }
        );
    }
}

public class Course
{
    public string CourseNumber { get; set; }
    public List<PriceGroup> PriceGroups { get; set; }
}

public class PriceGroup
{
    public int Id { get; set; }
    public string CourseNumber { get; set; }
}