EF Core 6 新功能汇总（三）
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 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 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_03EF.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(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 Posts { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity()
.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 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 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
{
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
{
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 Polish
class 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 Posts { get; set; }
}
class PostsByLanguage
{
public string Language { get; set; }
public int PostCount { get; set; }
}
class ExampleContext : DbContext
{
public DbSet Posts { get; set; }
public DbSet Blogs { get; set; }
public DbSet PostsByLanguages { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity()
.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
// on
class Person
{
public int Id { get; set; }
public string Name { get; set; }
}
class ExampleContext : DbContext
{
public DbSet 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 Posts { get; set; } = new List();
}
class Post
{
public int Id { get; set; }
public string Title { get; set; }
public Blog Blog { get; set; }
}
class ExampleContext : DbContext
{
public DbSet Blogs { get; set; }
public DbSet 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 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 Blogs { get; set; }
public DbSet 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 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:6
SELECT 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 People { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity()
.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 People { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity()
.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