如何使用Dapper映射嵌套对象列表

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，可以在多个位置教授一门课程。Entity Framework为我执行了映射，因此我的Course对象中填充了位置列表。我将如何使用Dapper做到这一点，甚至有可能还是我必须在几个查询步骤中做到这一点？

Dapper不是一个成熟的ORM，它不能处理神奇的查询等问题。

对于您的特定示例，以下可能会起作用：

抢修课程：

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

获取相关映射：

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

抓住相关地点

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

全部映射

将其留给读者，您将创建一些地图，然后遍历包含位置的课程。

请注意，in如果您的查询少于2100个（Sql Server），则可以使用该技巧，如果查询次数较多，则可能需要将查询修改select * from CourseLocations where CourseId in (select Id from Courses ... )为这种情况，那么您最好一次使用所有结果即可QueryMultiple

或者，您可以对查询使用一个查询：

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;

看到这里https://www.tritac.com/blog/dappernet-by-example/

无需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();

我知道我真的来晚了，但是还有另一种选择。您可以在此处使用QueryMultiple。像这样：

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();
}

很抱歉迟到聚会（像往常一样）。对我来说，它更容易使用Dictionary，就像吉荣ķ那样，在性能和可读性方面。另外，为了避免跨位置的标头乘法，我使用Distinct()了消除潜在的重复：

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;
}

缺了点什么。如果未Locations在SQL查询中指定每个字段，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.*在查询中使用的是位置列表，但没有数据。

不知道是否有人需要它，但是我有没有Model的动态版本，可以快速灵活地进行编码。

var lookup = new Dictionary<int, dynamic>();
conn.Query<dynamic, dynamic, dynamic>(@"
    SELECT A.*, B.*
    FROM Client A
    INNER JOIN Instance B ON A.ClientID = B.ClientID                
    ", (A, B) => {
        // If dict has no key, allocate new obj
        // with another level of array
        if (!lookup.ContainsKey(A.ClientID)) {
            lookup[A.ClientID] = new {
                ClientID = A.ClientID,
                ClientName = A.Name,                                        
                Instances = new List<dynamic>()
            };
        }

        // Add each instance                                
        lookup[A.ClientID].Instances.Add(new {
            InstanceName = B.Name,
            BaseURL = B.BaseURL,
            WebAppPath = B.WebAppPath
        });

        return lookup[A.ClientID];
    }, splitOn: "ClientID,InstanceID").AsQueryable();

var resultList = lookup.Values;
return resultList;