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：
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。Entity Framework 为我进行映射，因此我的 Course 对象填充了位置列表。我将如何使用 Dapper 来解决这个问题，甚至有可能还是我必须在几个查询步骤中完成它？
或者，您可以使用一个带有查找的查询：
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;
Dapper 不是一个成熟的 ORM，它不处理查询的神奇生成等。
对于您的特定示例，以下可能会起作用：
var courses = cnn.Query<Course>("select * from Courses where Category = 1 Order by CreationDate");
var mappings = cnn.Query<CourseLocation>(
"select * from CourseLocations where CourseId in @Ids",
new {Ids = courses.Select(c => c.Id).Distinct()});
var locations = cnn.Query<Location>(
"select * from Locations where Id in @Ids",
new {Ids = mappings.Select(m => m.LocationId).Distinct()}
);
将此留给读者，您创建一些地图并遍历您的课程填充位置。
警告in如果您的查找次数少于2100 （Sql Server），该技巧将起作用，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 from Courses ... )如果是这种情况，您不妨一次性使用所有结果QueryMultiple
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();
我知道我真的迟到了，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();
}
很抱歉迟到了（和往常一样）。对我来说，就性能和可读性而言，使用 a 更容易Dictionary，就像 Jeroen K 所做的那样。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;
}
缺了点什么。如果您未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;
在查询中使用l.*，我有位置列表但没有数据。
不确定是否有人需要它，但我有没有模型的动态版本，用于快速灵活的编码。
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;
还有另一种使用 JSON 结果的方法。尽管已接受的答案和其他答案得到了很好的解释，但我只是想到了另一种获得结果的方法。
创建存储过程或选择 qry 以 json 格式返回结果。然后将结果对象反序列化为所需的类格式。请通过示例代码。
using (var db = connection.OpenConnection())
{
var results = await db.QueryAsync("your_sp_name",..);
var result = results.FirstOrDefault();
string Json = result?.your_result_json_row;
if (!string.IsNullOrEmpty(Json))
{
List<Course> Courses= JsonConvert.DeserializeObject<List<Course>>(Json);
}
//map to your custom class and dto then return the result
}
这是另一个思考过程。请检查相同。