如何在Entity Framework Core中构建多个左连接查询
考虑我有以下实体：
public class Root { public long Id { get; set; } } public class School : Root { public long StudentId { get; set; } public Student Student { get; set; } public Teacher Teacher { get; set; } public long TeacherId { get; set; } } public class Student : Root { } public class Teacher : Root { } 现在，在EF中进行此修复后，我可以构建左连接查询，如下所示：
ctx.Schools .GroupJoin(ctx.Teachers, school => school.TeacherId, teacher => teacher.Id, (school, teachers) => new { school, teachers }) .SelectMany(info => info.teachers.DefaultIfEmpty(), (info, teacher) => new { info.school, teacher }) .Where(info => info.school.Id == someSchoolId) .Select(r => r.school);
或者像这样：
from school in ctx.Schools join teacher in ctx.Teachers on school.TeacherId equals teacher.Id into grouping from t in grouping.DefaultIfEmpty() where school.Id == someSchoolId select school;
生成的sql是：
SELECT [school].[Id], [school].[StudentId], [school].[TeacherId], [teacher].[Id] FROM [Schools] AS [school] LEFT JOIN [Teachers] AS [teacher] ON [school].[TeacherId] = [teacher].[Id] WHERE [school].[Id] = @__someSchoolId_0 ORDER BY [school].[TeacherId]
但是（！），当我尝试向左连接添加一个表时
ctx.Schools .GroupJoin(ctx.Teachers, school => school.TeacherId, teacher => teacher.Id, (school, teachers) => new { school, teachers }) .SelectMany(info => info.teachers.DefaultIfEmpty(), (info, teacher) => new { info.school, teacher }) .GroupJoin(ctx.Students, info => info.school.StudentId, student => student.Id, (info, students) => new {info.school, info.teacher, students}) .SelectMany(info => info.students.DefaultIfEmpty(), (info, student) => new {info.school, info.teacher, student}) .Where(data => data.school.Id == someSchoolId) .Select(r => r.school);
要么
from school in ctx.Schools join teacher in ctx.Teachers on school.TeacherId equals teacher.Id into grouping from t in grouping.DefaultIfEmpty() join student in ctx.Students on school.StudentId equals student.Id into grouping2 from s in grouping2.DefaultIfEmpty() where school.Id == someSchoolId select school;
生成了两个单独的SQL查询：
SELECT [student].[Id] FROM [Students] AS [student] SELECT [school].[Id], [school].[StudentId], [school].[TeacherId], [teacher].[Id] FROM [Schools] AS [school] LEFT JOIN [Teachers] AS [teacher] ON [school].[TeacherId] = [teacher].[Id] WHERE [school].[Id] = @__someSchoolId_0 ORDER BY [school].[TeacherId]
看起来有客户端左连接出现。
我究竟做错了什么？
您需要从所有 3个表中进行选择 ，以便当Entity Framework从Linq AST转换为SQL时， 左连接是有意义的
select new { school, t, s };
代替
select school;
然后，如果您在程序执行期间从Visual Studio检入Debug并将查询的值复制到剪贴板，您将找到 – 如预期的那样 – 在FROM之后的2个LEFT OUTER JOIN
勘误表
从EF 6可以看到2个左外连接。
EF Core记录器写入查询…
无法翻译，将在本地进行评估。
这里唯一的注意事项是 – 在没有选择其他表的情况下 – 首先没有理由找到多个左连接
EF核心设计
基于github repo中看到的unit testing并试图更接近满足OP要求，我建议以下查询
var querySO = ctx.Schools .Include(x => x.Student) .Include(x => x.Teacher) ; var results = querySO.ToArray();
这次我从EF Core Logger看到了几个LEFT JOIN
PRAGMA foreign_keys = ON;执行DbCommand（0ms）[Parameters = []，CommandType =’Text’，CommandTimeout = ’30’]
选择“x”。“SchoolId”，“x”。“StudentId”，“x”。“TeacherId”，“s”。“StudentId”，“s”。“name”，“t”。“TeacherId”，“ T”，‘名’
来自“学校”AS“x”
LEFT JOIN “Students”AS“s”ON“x”。“StudentId”=“s”。“StudentId”
LEFT JOIN “Teachers”AS“t”ON“x”。“TeacherId”=“t”。“TeacherId”
定义了一个模型
protected override void OnModelCreating(ModelBuilder modelBuilder) { base.OnModelCreating(modelBuilder); modelBuilder.Entity().HasKey(p => p.SchoolId); modelBuilder.Entity().HasKey(p => p.TeacherId); modelBuilder.Entity().HasKey(p => p.StudentId); modelBuilder.Entity().HasOne(s => s.Student) .WithOne().HasForeignKey(s => s.StudentId); modelBuilder.Entity ().HasOne(s => s.Teacher) .WithOne().HasForeignKey(s => s.TeacherId); }
和课程
public class School { public long SchoolId { get; set; } public long? StudentId { get; set; } public Student Student { get; set; } public Teacher Teacher { get; set; } public long? TeacherId { get; set; } } public class Student { public long StudentId { get; set; } public string name { get; set; } } public class Teacher { public long TeacherId { get; set; } public string name { get; set; } }