如何使用 Dapper 映射嵌套对象列表

问题描述

我目前正在使用实体框架来访问我的数据库，但想看看 Dapper.我有这样的课程:

I'm currently using Entity Framework for my db access but want to have a look at Dapper. I have classes like this:

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.实体框架为我做映射，所以我的 Course 对象填充了一个位置列表.我将如何使用 Dapper 解决这个问题，它甚至可能还是必须在多个查询步骤中完成?

So one course can be taught at several locations. Entity Framework does the mapping for me so my Course object is populated with a list of locations. How would I go about this with Dapper, is it even possible or do I have to do it in several query steps?

推荐答案

Dapper 不是一个完整的 ORM，它不处理神奇的查询生成等.

Dapper is not a full blown ORM it does not handle magic generation of queries and such.

对于您的特定示例，以下可能有效:

For your particular example the following would probably work:

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

抓取相关映射:

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

抓取相关位置

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

全部映射

将此留给读者，您可以创建一些地图并遍历包含位置的课程.

Map it all up

Leaving this to the reader, you create a few maps and iterate through your courses populating with the locations.

警告 in 技巧将起作用，如果您的数量少于 2100 个查找(Sql Server)，如果您有更多查找，您可能希望将查询修改为 select * from CourseLocations where CourseId in (select Id来自 Courses ... ) 如果是这种情况，您也可以使用 QueryMultiple

Caveat the in trick will work if you have less than 2100 lookups (Sql Server), if you have more you probably want to amend the query to select * from CourseLocations where CourseId in (select Id from Courses ... ) if that is the case you may as well yank all the results in one go using QueryMultiple

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