使用Dapper在Linq中进行多对多 [英] Many to Many in Linq using Dapper
问题描述
我有地方,每个地方可以有很多标签.每个标签都可以分配到很多地方.
I have Places, each place can have many tags. Each tag can be assigned to many places.
public class Place {
public int Id { get; set; }
public string PlaceName { get; set; }
public IEnumerable<Tag> Tags { get; set; }
}
public class Tag {
public int Id { get; set; }
public string TagName { get; set; }
}
public class TagPlace {
public int Id { get; set; }
public PlaceId { get; set; }
public TagId { get; set; }
}
数据库中有相应的带有外键的表.
The database has equivalent tables with foreign keys as appropriate.
我想获取一个地方集合,并且我希望每个地方都具有适当的标签集合.我猜可能需要使用Linq.
I want to get a collection of Places, and I want each Place to have an appropriate colleciton of Tags. I guess using Linq might be required.
我已经找到了很多关于此的文章,但是它们并不是完全相同/只处理一个int列表,而不是两个对象集合.
I've found various articles on this, but they aren't quite the same / deal with a list of ints rather than two collections of objects.
做到这一点的最好方法是什么?
What's the best way of doing this?
推荐答案
Dapper的经典方法是在查询枚举记录时使用字典来存储主要对象
The classical approach with Dapper is to use a Dictionary to store the main objects while the query enumerates the records
public IEnumerable<Place> SelectPlaces()
{
string query = @"SELECT p.id, p.PlaceName, t.id, t.tagname
FROM Place p INNER JOIN TagPlace tp ON tp.PlaceId = p.Id
INNER JOIN Tag t ON tp.TagId = t.Id";
var result = default(IEnumerable<Place>);
Dictionary<int, Place> lookup = new Dictionary<int, Place>();
using (IDbConnection connection = GetOpenedConnection())
{
// Each record is passed to the delegate where p is an instance of
// Place and t is an instance of Tag, delegate should return the Place instance.
result = connection.Query<Place, Tag, Place(query, (p, t) =>
{
// Check if we have already stored the Place in the dictionary
if (!lookup.TryGetValue(p.Id, out Place placeFound))
{
// The dictionary doesnt have that Place
// Add it to the dictionary and
// set the variable where we will add the Tag
lookup.Add(p.Id, p);
placeFound = p;
// Probably it is better to initialize the IEnumerable
// directly in the class
placeFound.Tags = new List<Tag>();
}
// Add the tag to the current Place.
placeFound.Tags.Add(t);
return placeFound;
}, splitOn: "id");
// SplitOn is where we tell Dapper how to split the record returned
// in the two instances required, but here SplitOn
// is not really needed because "Id" is the default.
}
return result;
}
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