在这篇文章中,我将重点介绍 EF Core 6 中 LINQ 查询功能的增强。
这是 EF Core 6 新功能汇总的第三篇文章:
EF Core 6 新功能汇总(一)
EF Core 6 新功能汇总(二)
EF Core 6 新功能汇总(三)
1对 GroupBy 查询的更好支持
EF Core 6.0 对 GroupBy
查询有更好的支持。
翻译
GroupBy
后面的FirstOrDefault
在
GroupBy
之后使用ThenBy
支持从一个组中选择前 N 个结果
using var context = new ExampleContext();
var query = context.People.GroupBy(p => p.FirstName).Select(g => g.OrderBy(e => e.FirstName).ThenBy(e => e.LastName).FirstOrDefault()).ToQueryString();
Console.WriteLine(query);class Person
{public int Id { get; set; }public string FirstName { get; set; }public int LastName { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6GroupBy");
}
翻译后的 SQL:
SELECT[t0].[Id], [t0].[FirstName], [t0].[LastName]
FROM (
SELECT[p].[FirstName]FROM [People] AS [p]GROUP BY [p].[FirstName]
) AS[t]
LEFT JOIN(SELECT[t1].[Id], [t1].[FirstName], [t1].[LastName]FROM (SELECT[p0].[Id], [p0].[FirstName], [p0].[LastName],ROW_NUMBER() OVER(PARTITION BY [p0].[FirstName]ORDER BY [p0].[FirstName], [p0].[LastName]) AS[row]FROM[People] AS[p0]) AS[t1]WHERE[t1].[row] <= 1
) AS[t0] ON[t].[FirstName] = [t0].[FirstName]
2三四个参数的 String.Concat 翻译
以前 EF Core 翻译 string.Concat
时只有两个参数。EF Core 6.0 支持翻译三个和四个参数的 string.Concat
。
using var context = new ExampleContext();
string fullName = "SamuelLanghorneClemens";
var query = context.Blogs.Where(b => string.Concat(b.FirstName, b.MiddleName, b.LastName) == fullName).ToQueryString();
Console.WriteLine(query);class Blog
{public int Id { get; set; }public string FirstName { get; set; }public string MiddleName { get; set; }public string LastName { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Blog> Blogs { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6StringConcat");
}
翻译后的 SQL:
DECLARE @__fullName_0 nvarchar(4000) = N'SamuelLanghorneClemens';SELECT[b].[Id], [b].[FirstName], [b].[LastName], [b].[MiddleName]
FROM[Blogs] AS[b]
WHERE(COALESCE([b].[FirstName], N'') + (COALESCE([b].[MiddleName], N'') +COALESCE([b].[LastName], N ''))) = @__fullName_0
3EF.Functions.FreeText 支持二进制列
以前,尽管 SQL FreeText
函数支持二进制列,但你不能在二进制列上使用 EF.Functions.FreeText
方法。EF Core 6.0 解决了这个问题。
using var context = new ExampleContext();
var query = context.Posts.Where(p => EF.Functions.FreeText(EF.Property<string>(p, "Content"), "Searching text")).ToQueryString();
Console.WriteLine(query);class Post
{public int Id { get; set; }public string Title { get; set; }public byte[] Content { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Post> Posts { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Post>().Property(x => x.Content).HasColumnType("varbinary(max)");}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6FlexibleTextSearch");
}
翻译后的 SQL:
SELECT "p"."Id", "p"."Name", "p"."PhoneNumber"
FROM "People" AS "p"
WHERE CAST("p"."PhoneNumber" AS TEXT) LIKE '%368%'
4EF.Functions.Random
EF Core 6.0 引入了一个新的 EF.Functions.Random
方法。它映射了 SQL 函数 RAND()
。已经实现了对 SQL Server、SQLite 和 Cosmos 的翻译。
using var context = new ExampleContext();
var query = context.Posts.Where(p => p.Rating == (int)(EF.Functions.Random() * 5.0) + 1).ToQueryString();
Console.WriteLine(query);class Post
{public int Id { get; set; }public string Title { get; set; }public int Rating { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Post> Posts { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6Random");
}
翻译后的 SQL:
SELECT[p].[Id], [p].[Rating], [p].[Title]
FROM[Posts] AS[p]
WHERE[p].[Rating] = (CAST((RAND() * 5.0E0) AS int) + 1)
5改进了 SQL Server 的 IsNullOrWhitespace 的翻译
以前,EF Core 将 string.IsNullOrWhiteSpace
翻译成在判断前将值进行 trim
操作。EF Core 6.0 已经不这么做了。
using var context = new ExampleContext();
var query = context.Entities.Where(e => string.IsNullOrWhiteSpace(e.Property)).ToQueryString();
Console.WriteLine(query);class Entity
{public int Id { get; set; }public string Property { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Entity> Entities { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6IsNullOrWhiteSpace");
}
以前翻译的 SQL:
SELECT [e].[Id], [e].[Property]
FROM [Entities] AS[e]
WHERE [e].[Property] IS NULL OR (LTRIM(RTRIM([e].[Property])) = N'')
现在翻译的 SQL:
SELECT [e].[Id], [e].[Property]
FROM [Entities] AS[e]
WHERE [e].[Property] IS NULL OR ([e].[Property] = N'')
6为内存数据库定义查询
在 EF Core 6.0 中,你可以通过一个新的方法 ToInMemoryQuery
来定义一个针对内存数据库的查询。这对于创建内存数据库的视图是最有用的。
using var context = new ExampleContext();
var blogEn = new Blog
{Title = "All about .NET",Language = "English",Posts = new List<Post>{new Post { Title = "Post one", Content = "Some content" },new Post { Title = "Post two", Content = "Some content" }}
};
var blogPl = new Blog
{Title = "Wszystko o .NET",Language = "Polish",Posts = new List<Post>{new Post { Title = "Pierwszy post", Content = "Treść" }}
};
context.Blogs.Add(blogEn);
context.Blogs.Add(blogPl);
await context.SaveChangesAsync();var postsByLanguages = context.PostsByLanguages.ToList();
postsByLanguages.ForEach(p => Console.WriteLine($"{p.PostCount} posts in {p.Language}"));
// Output:
// 2 posts in English
// 1 posts in Polishclass Post
{public int Id { get; set; }public string Title { get; set; }public string Content { get; set; }
}
class Blog
{public int Id { get; set; }public string Title { get; set; }public string Language { get; set; }public ICollection<Post> Posts { get; set; }
}
class PostsByLanguage
{public string Language { get; set; }public int PostCount { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Post> Posts { get; set; }public DbSet<Blog> Blogs { get; set; }public DbSet<PostsByLanguage> PostsByLanguages { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<PostsByLanguage>().HasNoKey().ToInMemoryQuery(() => Blogs.GroupBy(c => c.Language).Select(g =>new PostsByLanguage{Language = g.Key,PostCount = g.Sum(b => b.Posts.Count)}));}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseInMemoryDatabase("ToInMemoryQuery");
}
7单参数的 Substring 翻译
以前,EF Core 只翻译有两个参数的 string.Substring
重载。EF Core 6.0 支持翻译单个参数的 string.Substring
。
using var context = new ExampleContext();
context.People.Add(new Person { Name = "John" });
context.People.Add(new Person { Name = "Bred" });
context.People.Add(new Person { Name = "Ron" });
await context.SaveChangesAsync();var result = await context.People.Select(a => new { Name = a.Name.Substring(1) }).ToListAsync();
result.ForEach(p => Console.WriteLine(p.Name));
// Output:
// ohn
// red
// onclass Person
{public int Id { get; set; }public string Name { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6Substring");
}
翻译后的 SQL:
SELECT SUBSTRING([p].[Name], 1 + 1, LEN([p].[Name])) AS [Name]
FROM [People] AS [p]
8非导航集合的分割查询
EF Core 支持将一个 LINQ 查询拆分成多个 SQL 查询。EF Core 6.0 可以分割一个 LINQ 查询,其中非导航集合属性包含在查询投影中。
using var context = new ExampleContext();
var blog = new Blog { Name = ".NET Blog"};
blog.Posts.Add(new Post { Title = "First .NET post" });
blog.Posts.Add(new Post { Title = "Second Java post" });
blog.Posts.Add(new Post { Title = "Third .NET post" });
context.Blogs.Add(blog);
await context.SaveChangesAsync();var blogsWithDotnetPosts = await context.Blogs.Select(b => new{b,Posts = b.Posts.Where(p => p.Title.Contains(".NET")),}).AsSplitQuery().ToListAsync();class Blog
{public int Id { get; set; }public string Name { get; set; }public ICollection<Post> Posts { get; set; } = new List<Post>();
}
class Post
{public int Id { get; set; }public string Title { get; set; }public Blog Blog { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Blog> Blogs { get; set; }public DbSet<Post> Posts { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6SplitQueries");
}
单个 SQL 查询(不用 AsSplitQuery
):
SELECT [b].[Id], [b].[Name], [t].[BlogId], [t].[Title]
FROM [Blogs] AS [b]
LEFT JOIN (SELECT [p].[Id], [p].[BlogId], [p].[Title]FROM [Posts] AS [p]WHERE [p].[Title] LIKE N'%.NET%'
) AS [t] ON [b].[Id] = [t].[BlogId]
ORDER BY [b].[Id]
多个 SQL 查询(使用了 AsSplitQuery
):
SELECT [b].[Id], [b].[Name]
FROM [Blogs] AS [b]
ORDER BY [b].[Id]SELECT [t].[Id], [t].[BlogId], [t].[Title], [b].[Id]
FROM [Blogs] AS [b]
INNER JOIN (SELECT [p].[Id], [p].[BlogId], [p].[Title]FROM [Posts] AS [p]WHERE [p].[Title] LIKE N'%.NET%'
) AS [t] ON [b].[Id] = [t].[BlogId]
ORDER BY [b].[Id]
9删除最后的 ORDER BY 子句
当连接相关实体时,EF Core 添加了 ORDER BY 子句,以确保给定实体的所有相关实体被分组。然而,最后一个子句并不是必须的,而且会对性能产生影响。EF Core 6.0 删除了它。
using var context = new ExampleContext();
var query = context.Blogs.Include(b => b.Posts.Where(p => p.Rating > 3)).ToQueryString();
Console.WriteLine(query);class Blog
{public int Id { get; set; }public string Name { get; set; }public ICollection<Post> Posts { get; set; }
}
class Post
{public int Id { get; set; }public string Title { get; set; }public int Rating { get; set; }public Blog Blog { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Blog> Blogs { get; set; }public DbSet<Post> Posts { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6RemoveLastOrderByClause");
}
EF Core 5.0 翻译的 SQL:
SELECT [b].[Id], [b].[Name], [t].[Id], [t].[BlogId], [t].[Rating], [t].[Title]
FROM [Blogs] AS [b]
LEFT JOIN (SELECT [p].[Id], [p].[BlogId], [p].[Rating], [p].[Title]FROM [Posts] AS [p]WHERE [p].[Rating] > 3
) AS [t] ON [b].[Id] = [t].[BlogId]
ORDER BY [b].[Id], [t].[Id]
EF Core 6.0 翻译的 SQL:
SELECT [b].[Id], [b].[Name], [t].[Id], [t].[BlogId], [t].[Rating], [t].[Title]
FROM [Blogs] AS [b]
LEFT JOIN (SELECT [p].[Id], [p].[BlogId], [p].[Rating], [p].[Title]FROM [Posts] AS [p]WHERE [p].[Rating] > 3
) AS [t] ON [b].[Id] = [t].[BlogId]
ORDER BY [b].[Id]
10用文件名和行号标记查询
从 EF Core 2.2 开始,你可以给你的查询添加一个标签,以达到更好的调试目的。EF Core 6.0 更进一步,现在你可以用 LINQ 代码的文件名和行号来标记查询。
using var context = new ExampleContext();
var query = context.Blogs.TagWithCallSite().OrderBy(b => b.CreationDate).Take(10).ToQueryString();
Console.WriteLine(query);class Blog
{public int Id { get; set; }public string Name { get; set; }public DateTime CreationDate { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Blog> Blogs { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6TagWithCallSite");
}
翻译后的 SQL:
DECLARE @__p_0 int = 10;--File: D:\EFCore6\TagWithCallSite\TagWithCallSite\Program.cs:6SELECT TOP(@__p_0) [b].[Id], [b].[CreationDate], [b].[Name]
FROM[Blogs] AS[b]
ORDER BY[b].[CreationDate]
11自有可选从属关系处理
EF Core 6.0 改变了一些对自有可选从属关系的处理。当一个模型有自己的可选从属关系时,EF Core 会在你保存它时警告你所有缺失的属性。
using var context = new ExampleContext();
var person = new Person
{FirstName = "Oleg",LastName = "Kyrylchuk",Address = new Address()
};
context.People.Add(person);
await context.SaveChangesAsync();class Person
{public int Id { get; set; }public string FirstName { get; set; }public string LastName { get; set; }public Address Address { get; set; }
}
class Address
{public string City { get; set; }public string Street { get; set; }public string PostalCode { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Person>().OwnsOne(p => p.Address);}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.EnableSensitiveDataLogging().LogTo(Console.WriteLine).UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6OwnedDependentHandling");
}
警告日志:
当你有嵌套自有可选从属关系时,EF Core 将不允许创建模型。
using var context = new ExampleContext();
var person = new Person
{FirstName = "Oleg",LastName = "Kyrylchuk",ContactInfo = new ContactInfo()
};
context.People.Add(person);
await context.SaveChangesAsync();class Person
{public int Id { get; set; }public string FirstName { get; set; }public string LastName { get; set; }public ContactInfo ContactInfo { get; set; }
}
class ContactInfo
{public string Phone { get; set; }public Address Address { get; set; }
}
class Address
{public string City { get; set; }public string Street { get; set; }public string PostalCode { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Person>().OwnsOne(p => p.ContactInfo).OwnsOne(p => p.Address);}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6OwnedDependentHandling");
}
创建模型后将会抛出异常。
这些变化迫使你避免这种情况。你可以通过以下方式解决这些问题。
使从属关系成为必需的。
在从属关系中至少有一个必需属性。
为可选的从属关系创建自己的表,而不是与主体共享它们。
12结尾
本文所有代码示例都可以在我的 GitHub 中找到:
https://github.com/okyrylchuk/dotnet6_features/tree/main/EF%20Core%206#linq-query-enhancements
原文:bit.ly/32DqXnu
作者:Oleg Kyrylchuk
翻译:精致码农