如何使用Dapper映射嵌套对象的列表

Question

我目前正在使用Entity Framework进行数据库访问,但想看看Dapper.我有这样的课程:

public class Course{
   public string Title{get;set;}
   public IList<Location> Locations {get;set;}
   ...
}

public class Location{
   public string Name {get;set;}
   ...
}

因此,可以在多个地点教授一门课程.实体框架为我执行映射,因此我的Course对象填充了一个位置列表.我如何与Dapper一起讨论这个问题,它是否可能,或者我必须在几个查询步骤中执行此操作？

Answer 1

或者,您可以将一个查询与查找一起使用:

var lookup = new Dictionary<int, Course>();
conn.Query<Course, Location, Course>(@"
    SELECT c.*, l.*
    FROM Course c
    INNER JOIN Location l ON c.LocationId = l.Id                    
    ", (c, l) => {
        Course course;
        if (!lookup.TryGetValue(c.Id, out course))
            lookup.Add(c.Id, course = c);
        if (course.Locations == null) 
            course.Locations = new List<Location>();
        course.Locations.Add(l); /* Add locations to course */
        return course;
     }).AsQueryable();
var resultList = lookup.Values;

请参见https://www.tritac.com/blog/dappernet-by-example/

Answer 2

Dapper并不是一个完整的ORM它不会处理魔法生成的查询等.

对于您的特定示例,以下可能会起作用:

抓住课程:

var courses = cnn.Query<Course>("select * from Courses where Category = 1 Order by CreationDate");

抓住相关的映射:

var mappings = cnn.Query<CourseLocation>(
   "select * from CourseLocations where CourseId in @Ids", 
    new {Ids = courses.Select(c => c.Id).Distinct()});

抓住相关位置

var locations = cnn.Query<Location>(
   "select * from Locations where Id in @Ids",
   new {Ids = mappings.Select(m => m.LocationId).Distinct()}
);

全部映射

将此信息留给读者,您可以创建一些地图并遍历填充位置的课程.

需要注意的in伎俩将工作,如果你有小于2100个查询(SQL Server)的,如果你有更多你可能想查询修改为select * from CourseLocations where CourseId in (select Id from Courses ... )如果这是你可以使用以及抽出一个全力以赴的结果的情况下QueryMultiple

Answer 3

不需要lookup字典

var coursesWithLocations = 
    conn.Query<Course, Location, Course>(@"
        SELECT c.*, l.*
        FROM Course c
        INNER JOIN Location l ON c.LocationId = l.Id                    
        ", (course, location) => {
            course.Locations = course.Locations ?? new List<Location>();
            course.Locations.Add(location); 
            return course;
        }).AsQueryable();

Answer 4

我知道我已经迟到了,但还有另一种选择.您可以在此处使用QueryMultiple.像这样的东西:

var results = cnn.QueryMultiple(@"
    SELECT * 
      FROM Courses 
     WHERE Category = 1 
  ORDER BY CreationDate
          ; 
    SELECT A.*
          ,B.CourseId 
      FROM Locations A 
INNER JOIN CourseLocations B 
        ON A.LocationId = B.LocationId 
INNER JOIN Course C 
        ON B.CourseId = B.CourseId 
       AND C.Category = 1
");

var courses = results.Read<Course>();
var locations = results.Read<Location>(); //(Location will have that extra CourseId on it for the next part)
foreach (var course in courses) {
   course.Locations = locations.Where(a => a.CourseId == course.CourseId).ToList();
}

Answer 5

很抱歉迟到了（像往常一样）。对我来说，在性能和可读性方面Dictionary，像 Jeroen K 那样使用 更容易。此外，为了避免跨位置的标题乘法，我使用Distinct()删除潜在的重复：

string query = @"SELECT c.*, l.*
    FROM Course c
    INNER JOIN Location l ON c.LocationId = l.Id";
using (SqlConnection conn = DB.getConnection())
{
    conn.Open();
    var courseDictionary = new Dictionary<Guid, Course>();
    var list = conn.Query<Course, Location, Course>(
        query,
        (course, location) =>
        {
            if (!courseDictionary.TryGetValue(course.Id, out Course courseEntry))
            {
                courseEntry = course;
                courseEntry.Locations = courseEntry.Locations ?? new List<Location>();
                courseDictionary.Add(courseEntry.Id, courseEntry);
            }

            courseEntry.Locations.Add(location);
            return courseEntry;
        },
        splitOn: "Id")
    .Distinct()
    .ToList();

    return list;
}

Answer 6

缺了点什么。如果不指定LocationsSQL 查询中的每个字段，Location则无法填充对象。看一看：

var lookup = new Dictionary<int, Course>()
conn.Query<Course, Location, Course>(@"
    SELECT c.*, l.Name, l.otherField, l.secondField
    FROM Course c
    INNER JOIN Location l ON c.LocationId = l.Id                    
    ", (c, l) => {
        Course course;
        if (!lookup.TryGetValue(c.Id, out course)) {
            lookup.Add(c.Id, course = c);
        }
        if (course.Locations == null) 
            course.Locations = new List<Location>();
        course.Locations.Add(a);
        return course;
     },
     ).AsQueryable();
var resultList = lookup.Values;

l.*在查询中使用时，我有位置列表但没有数据。