EF Core 6 新功能彙總（一）

在這篇文章中，你將看到 EF Core 6 中的十個新功能，包括新的屬性標記、對時態表、稀疏欄位的支援，以及其他新功能。

1 Unicode 屬性

在 EF Core 6.0 中，新的 UnicodeAttribute 允許你將一個字串屬性對應到一個非 Unicode 欄位，而不需要直接指定資料庫型別。當資料庫系統只支援 Unicode 型別時，Unicode 屬性會被忽略。

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; }

    [Unicode(false)]
    [MaxLength(22)]
    public string Isbn { get; set; }
}

對應的遷移程式碼：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Books",
        columns: table => new
        {
            Id = table.Column<int>(type: "int", nullable: false)
                        .Annotation("SqlServer:Identity", "1, 1"),
            Title = table.Column<string>(type: "nvarchar(max)", nullable: true),
            Isbn = table.Column<string>(type: "varchar(22)", unicode: false, maxLength: 22, nullable: true)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Books", x => x.Id);
        });
}

2 Precision 屬性

在 EF Core 6.0 之前，你可以用 Fluent API 設定精確度。現在，你也可以用資料標記和一個新的 PrecisionAttribute 來做這件事。

public class Product
{
    public int Id { get; set; }

    [Precision(precision: 10, scale: 2)]
    public decimal Price { get; set; }
}

對應的遷移程式碼：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Products",
        columns: table => new
        {
            Id = table.Column<int>(type: "int", nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            Price = table.Column<decimal>(type: "decimal(10,2)", precision: 10, scale: 2, nullable: false)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Products", x => x.Id);
        });
}

3 EntityTypeConfiguration 屬性

從 EF Core 6.0 開始，你可以在實體型別上放置一個新的 EntityTypeConfiguration 屬性，這樣 EF Core 就可以找到並使用適當的設定。在此之前，類別的設定必須被實例化並從 OnModelCreating 方法中呼叫。

public class ProductConfiguration : IEntityTypeConfiguration<Product>
{
    public void Configure(EntityTypeBuilder<Product> builder)
    {
        builder.Property(p => p.Name).HasMaxLength(250);
        builder.Property(p => p.Price).HasPrecision(10, 2);
    }
}
[EntityTypeConfiguration(typeof(ProductConfiguration))]
public class Product
{
    public int Id { get; set; }
    public decimal Price { get; set; }
    public string Name { get; set; }
}

4 Column 屬性

當你在模型中使用繼承時，你可能不滿意建立的表中預設的 EF Core 欄位順序。在 EF Core 6.0 中，你可以用 ColumnAttribute 指定欄位的順序。

此外，你還可以使用新的 Fluent API--HasColumnOrder() 來實現。

public class EntityBase
{
    [Column(Order = 1)]
    public int Id { get; set; }
    [Column(Order = 99)]
    public DateTime UpdatedOn { get; set; }
    [Column(Order = 98)]
    public DateTime CreatedOn { get; set; }
}
public class Person : EntityBase
{
    [Column(Order = 2)]
    public string FirstName { get; set; }
    [Column(Order = 3)]
    public string LastName { get; set; }
    public ContactInfo ContactInfo { get; set; }
}
public class Employee : Person
{
    [Column(Order = 4)]
    public string Position { get; set; }
    [Column(Order = 5)]
    public string Department { get; set; }
}
[Owned]
public class ContactInfo
{
    [Column(Order = 10)]
    public string Email { get; set; }
    [Column(Order = 11)]
    public string Phone { get; set; }
}

對應的遷移程式碼：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Employees",
        columns: table => new
        {
            Id = table.Column<int>(type: "int", nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            FirstName = table.Column<string>(type: "nvarchar(max)", nullable: true),
            LastName = table.Column<string>(type: "nvarchar(max)", nullable: true),
            Position = table.Column<string>(type: "nvarchar(max)", nullable: true),
            Department = table.Column<string>(type: "nvarchar(max)", nullable: true),
            ContactInfo_Email = table.Column<string>(type: "nvarchar(max)", nullable: true),
            ContactInfo_Phone = table.Column<string>(type: "nvarchar(max)", nullable: true),
            CreatedOn = table.Column<DateTime>(type: "datetime2", nullable: false),
            UpdatedOn = table.Column<DateTime>(type: "datetime2", nullable: false)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Employees", x => x.Id);
        });
}

5 時態表

EF Core 6.0 支援 SQL Server 的時態表。一個表可以設定成一個具有 SQL Server 預設的時間戳和歷史表的時態表。

public class ExampleContext : DbContext
{
    public DbSet<Person> People { get; set; }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder
            .Entity<Person>()
            .ToTable("People", b => b.IsTemporal());
    }
    protected override void OnConfiguring(DbContextOptionsBuilder options)
            => options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=TemporalTables;Trusted_Connection=True;");
}
public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
}

對應的遷移程式碼：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "People",
        columns: table => new
        {
            Id = table.Column<int>(type: "int", nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            Name = table.Column<string>(type: "nvarchar(max)", nullable: true),
            PeriodEnd = table.Column<DateTime>(type: "datetime2", nullable: false)
                .Annotation("SqlServer:IsTemporal", true)
                .Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd")
                .Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart"),
            PeriodStart = table.Column<DateTime>(type: "datetime2", nullable: false)
                .Annotation("SqlServer:IsTemporal", true)
                .Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd")
                .Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart")
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_People", x => x.Id);
        })
        .Annotation("SqlServer:IsTemporal", true)
        .Annotation("SqlServer:TemporalHistoryTableName", "PersonHistory")
        .Annotation("SqlServer:TemporalHistoryTableSchema", null)
        .Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd")
        .Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart");
}

你可以用以下方法查詢和檢索歷史資料：

• TemporalAsOf
• TemporalAll
• TemporalFromTo
• TemporalBetween
• TemporalContainedIn

使用時態表：

using ExampleContext context = new();
context.People.Add(new() { Name = "Oleg" });
context.People.Add(new() { Name = "Steve" });
context.People.Add(new() { Name = "John" });
await context.SaveChangesAsync();

var people = await context.People.ToListAsync();
foreach (var person in people)
{
    var personEntry = context.Entry(person);
    var validFrom = personEntry.Property<DateTime>("PeriodStart").CurrentValue;
    var validTo = personEntry.Property<DateTime>("PeriodEnd").CurrentValue;
    Console.WriteLine($"Person {person.Name} valid from {validFrom} to {validTo}");
}
// Output:
// Person Oleg valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM
// Person Steve valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM
// Person John valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM

查詢歷史資料：

var oleg = await context.People.FirstAsync(x => x.Name == "Oleg");
context.People.Remove(oleg);
await context.SaveChangesAsync();
var history = context
    .People
    .TemporalAll()
    .Where(e => e.Name == "Oleg")
    .OrderBy(e => EF.Property<DateTime>(e, "PeriodStart"))
    .Select(
        p => new
        {
            Person = p,
            PeriodStart = EF.Property<DateTime>(p, "PeriodStart"),
            PeriodEnd = EF.Property<DateTime>(p, "PeriodEnd")
        })
    .ToList();
foreach (var pointInTime in history)
{
    Console.WriteLine(
        $"Person {pointInTime.Person.Name} existed from {pointInTime.PeriodStart} to {pointInTime.PeriodEnd}");
}

// Output:
// Person Oleg existed from 06-Nov-21 17:50:39 PM to 06-Nov-21 18:11:29 PM

檢索歷史資料：

var removedOleg = await context
    .People
    .TemporalAsOf(history.First().PeriodStart)
    .SingleAsync(e => e.Name == "Oleg");

Console.WriteLine($"Id = {removedOleg.Id}; Name = {removedOleg.Name}");
// Output:
// Id = 1; Name = Oleg

了解更多關於時態表的資訊：https://devblogs.microsoft.com/dotnet/prime-your-flux-capacitor-sql-server-temporal-tables-in-ef-core-6-0/

6 稀疏欄位

EF Core 6.0 支援 SQL Server 稀疏欄位。在使用 TPH（table per hierarchy）繼承對映時，它可能很有用。

public class ExampleContext : DbContext
{
    public DbSet<Person> People { get; set; }
    public DbSet<Employee> Employees { get; set; }
    public DbSet<User> Users { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder
            .Entity<User>()
            .Property(e => e.Login)
            .IsSparse();

        modelBuilder
            .Entity<Employee>()
            .Property(e => e.Position)
            .IsSparse();
    }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
            => options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=SparseColumns;Trusted_Connection=True;");
}

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
}
public class User : Person
{
    public string Login { get; set; }
}
public class Employee : Person
{
    public string Position { get; set; }
}

對應遷移程式碼：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "People",
        columns: table => new
        {
            Id = table.Column<int>(type: "int", nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            Name = table.Column<string>(type: "nvarchar(max)", nullable: false),
            Discriminator = table.Column<string>(type: "nvarchar(max)", nullable: false),
            Position = table.Column<string>(type: "nvarchar(max)", nullable: true)
                .Annotation("SqlServer:Sparse", true),
            Login = table.Column<string>(type: "nvarchar(max)", nullable: true)
                .Annotation("SqlServer:Sparse", true)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_People", x => x.Id);
        });
}

稀疏欄位有限制，具體請看文件：https://docs.microsoft.com/en-us/sql/relational-databases/tables/use-sparse-columns?view=sql-server-ver15

7 EF Core 中的最小 API

EF Core 6.0 有它自己的最小 API。新的擴充方法可在同一行程式碼註冊一個 DbContext 型別，並提供一個資料庫 Provider 的設定。

const string AccountKey = "[CosmosKey]";

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSqlServer<MyDbContext>(@"Server = (localdb)\mssqllocaldb; Database = MyDatabase");

// OR
builder.Services.AddSqlite<MyDbContext>("Data Source=mydatabase.db");

// OR
builder.Services.AddCosmos<MyDbContext>($"AccountEndpoint=https://localhost:8081/;AccountKey={AccountKey}", "MyDatabase");

var app = builder.Build();
app.Run();

class MyDbContext : DbContext
{ }

8 遷移套件

在 EF Core 6.0 中，有一個新的有利於 DevOps 的功能--遷移套件。它允許建立一個包含遷移的小型可執行程式。你可以在 CD 中使用它。不需要複製原始碼或安裝 .NET SDK（只有執行階段）。

CLI：

dotnet ef migrations bundle --project MigrationBundles

Package Manager Console：

Bundle-Migration

更多介紹：https://devblogs.microsoft.com/dotnet/introducing-devops-friendly-ef-core-migration-bundles/

9 預設模型設定

EF Core 6.0 引入了一個預設模型設定。它允許你為一個給定的型別指定一次對映設定。例如，在處理值物件時，它可能很有幫助。

public class ExampleContext : DbContext
{
    public DbSet<Person> People { get; set; }
    public DbSet<Product> Products { get; set; }

    protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
    {
        configurationBuilder
            .Properties<string>()
            .HaveMaxLength(500);

        configurationBuilder
            .Properties<DateTime>()
            .HaveConversion<long>();

        configurationBuilder
            .Properties<decimal>()
            .HavePrecision(12, 2);

        configurationBuilder
            .Properties<Address>()
            .HaveConversion<AddressConverter>();
    }
}
public class Product
{
    public int Id { get; set; }
    public decimal Price { get; set; }
}
public class Person
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }
    public Address Address { get; set; }
}
public class Address
{
    public string Country { get; set; }
    public string Street { get; set; }
    public string ZipCode { get; set; }
}
public class AddressConverter : ValueConverter<Address, string>
{
    public AddressConverter()
        : base(
            v => JsonSerializer.Serialize(v, (JsonSerializerOptions)null),
            v => JsonSerializer.Deserialize<Address>(v, (JsonSerializerOptions)null))
    {
    }
}

10 已編譯模型

在 EF Core 6.0 中，你可以產生已編譯的模型（compiled models）。當你有一個大的模型，而你的 EF Core 啟動很慢時，這個功能是有意義的。你可以使用 CLI 或套件管理器主控台來做。

public class ExampleContext : DbContext
{
    public DbSet<Person> People { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
    {
        options.UseModel(CompiledModelsExample.ExampleContextModel.Instance)
        options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=SparseColumns;Trusted_Connection=True;");
    }
}
public class Person
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

CLI：

dotnet ef dbcontext optimize -c ExampleContext -o CompliledModels -n CompiledModelsExample

Package Manager Console：

Optimize-DbContext -Context ExampleContext -OutputDir CompiledModels -Namespace CompiledModelsExample

更多關於已編譯模型及其限制的介紹：

https://devblogs.microsoft.com/dotnet/announcing-entity-framework-core-6-0-preview-5-compiled-models/
https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-6.0/whatsnew#limitations

11 結尾

你可以在我的 GitHub 找到本文所有範例程式碼：https://github.com/okyrylchuk/dotnet6_features/tree/main/EF%20Core%206#miscellaneous-enhancements

原文：https://blog.okyrylchuk.dev/entity-framework-core-6-features-part-1

作者：Oleg Kyrylchuk

翻譯：精緻碼農