efcore-patterns

Entity Framework Core 模式

何时使用此技能

在以下情况使用此技能：

• 在新项目中设置 EF Core
• 优化查询性能
• 管理数据库迁移
• 将 EF Core 与 .NET Aspire 集成
• 调试更改跟踪问题
• 高效加载多个导航集合（查询拆分）

核心原则

1. 默认使用 NoTracking - 大多数查询都是只读的；需要跟踪时再启用
2. 绝不要手动编辑迁移 - 始终使用 CLI 命令
3. 专用迁移服务 - 将迁移执行与应用启动分离
4. 使用 ExecutionStrategy 进行重试 - 处理数据库瞬时故障
5. 显式更新 - 使用 NoTracking 时，显式将实体标记为待更新

---

模式 1：默认使用 NoTracking

配置 DbContext，默认禁用更改跟踪。这可以提升读操作密集型工作负载的性能。

public class ApplicationDbContext : DbContext
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
        : base(options)
    {
        // Disable change tracking by default for better performance on read-only queries
        // Use .AsTracking() explicitly for queries that need to track changes
        ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
    }

    public DbSet<Order> Orders => Set<Order>();
    public DbSet<Customer> Customers => Set<Customer>();
}

NoTracking 生效时

只读查询可正常运行：

// ✅ Fast read - no tracking overhead
var orders = await dbContext.Orders
    .Where(o => o.Status == OrderStatus.Pending)
    .ToListAsync();

写入操作需要显式处理：

// ❌ WRONG - Entity not tracked, SaveChanges does nothing
var order = await dbContext.Orders.FirstOrDefaultAsync(o => o.Id == orderId);
order.Status = OrderStatus.Shipped;
await dbContext.SaveChangesAsync(); // Nothing happens!

// ✅ CORRECT - Explicitly mark entity for update
var order = await dbContext.Orders.FirstOrDefaultAsync(o => o.Id == orderId);
order.Status = OrderStatus.Shipped;
dbContext.Orders.Update(order); // Marks entire entity as modified
await dbContext.SaveChangesAsync();

// ✅ ALSO CORRECT - Use AsTracking() for the query
var order = await dbContext.Orders
    .AsTracking()
    .FirstOrDefaultAsync(o => o.Id == orderId);
order.Status = OrderStatus.Shipped;
await dbContext.SaveChangesAsync(); // Works!

何时使用跟踪

场景	使用跟踪？	原因
在 UI 中显示数据	否	只读，无需更新
API GET 端点	否	返回数据，不进行修改
更新单个实体	是或显式调用 Update()	需要保存更改
包含导航属性的复杂更新	是	跟踪会处理关系
批量操作	否 + ExecuteUpdate	效率更高

显式添加/更新模式

public class OrderService
{
    private readonly ApplicationDbContext _db;

    // CREATE - Always use Add (works regardless of tracking)
    public async Task<Order> CreateOrderAsync(Order order)
    {
        _db.Orders.Add(order);
        await _db.SaveChangesAsync();
        return order;
    }

    // UPDATE - Explicitly mark as modified
    public async Task UpdateOrderStatusAsync(Guid orderId, OrderStatus newStatus)
    {
        var order = await _db.Orders.FirstOrDefaultAsync(o => o.Id == orderId)
            ?? throw new NotFoundException($"Order {orderId} not found");

        order.Status = newStatus;
        order.UpdatedAt = DateTimeOffset.UtcNow;

        // Explicitly mark as modified since DbContext uses NoTracking by default
        _db.Orders.Update(order);
        await _db.SaveChangesAsync();
    }

    // DELETE - Attach and remove
    public async Task DeleteOrderAsync(Guid orderId)
    {
        var order = new Order { Id = orderId };
        _db.Orders.Remove(order);
        await _db.SaveChangesAsync();
    }
}

---

模式 2：绝不要手动编辑迁移

重要： 始终使用 EF Core CLI 命令管理迁移。绝不要：

• 手动编辑迁移文件（在 Up()/Down() 中编写自定义 SQL 除外）
• 直接删除迁移文件
• 重命名迁移文件
• 在项目之间复制迁移

创建迁移

# Create a new migration
dotnet ef migrations add AddCustomerTable \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

# With a specific DbContext (if you have multiple)
dotnet ef migrations add AddCustomerTable \
    --context ApplicationDbContext \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

删除迁移

# Remove the last migration (if not yet applied)
dotnet ef migrations remove \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

# NEVER do this:
# rm Migrations/20240101_AddCustomerTable.cs  # ❌ BAD!

应用迁移

# Apply all pending migrations
dotnet ef database update \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

# Apply to a specific migration
dotnet ef database update AddCustomerTable \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

# Rollback to a previous migration
dotnet ef database update PreviousMigrationName \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

生成 SQL 脚本

# Generate SQL script for all migrations
dotnet ef migrations script \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api \
    --output migrations.sql

# Generate idempotent script (safe to run multiple times)
dotnet ef migrations script \
    --idempotent \
    --project src/MyApp.Infrastructure \
    --startup-project src/MyApp.Api

---

模式 3：使用 Aspire 的专用迁移服务

使用专用迁移服务，将迁移执行与主应用分离。这样可以确保：

• 在应用启动前完成迁移
• 清晰划分关注点
• 在测试环境中有序执行数据填充

项目结构

src/
├── MyApp.AppHost/           # Aspire orchestration
├── MyApp.Api/               # Main application
├── MyApp.Infrastructure/    # DbContext and migrations
└── MyApp.MigrationService/  # Dedicated migration runner

MigrationService Program.cs

using MyApp.Infrastructure.Data;
using MyApp.MigrationService;
using Microsoft.EntityFrameworkCore;

var builder = Host.CreateApplicationBuilder(args);

// Add Aspire service defaults
builder.AddServiceDefaults();

// Add PostgreSQL DbContext
var connectionString = builder.Configuration.GetConnectionString("appdb")
    ?? throw new InvalidOperationException("Connection string 'appdb' not found.");

builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseNpgsql(connectionString, npgsqlOptions =>
        npgsqlOptions.MigrationsAssembly("MyApp.Infrastructure")));

// Add the migration worker
builder.Services.AddHostedService<MigrationWorker>();

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

MigrationWorker.cs

public class MigrationWorker : BackgroundService
{
    private readonly IServiceProvider _serviceProvider;
    private readonly IHostApplicationLifetime _hostApplicationLifetime;
    private readonly ILogger<MigrationWorker> _logger;

    public MigrationWorker(
        IServiceProvider serviceProvider,
        IHostApplicationLifetime hostApplicationLifetime,
        ILogger<MigrationWorker> logger)
    {
        _serviceProvider = serviceProvider;
        _hostApplicationLifetime = hostApplicationLifetime;
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Migration service starting...");

        try
        {
            using var scope = _serviceProvider.CreateScope();
            var dbContext = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();

            await RunMigrationsAsync(dbContext, stoppingToken);

            _logger.LogInformation("Migration service completed successfully.");
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Migration service failed: {Error}", ex.Message);
            throw;
        }
        finally
        {
            // Stop the application after migrations complete
            _hostApplicationLifetime.StopApplication();
        }
    }

    private async Task RunMigrationsAsync(ApplicationDbContext dbContext, CancellationToken ct)
    {
        // Use execution strategy for transient failure handling
        var strategy = dbContext.Database.CreateExecutionStrategy();

        await strategy.ExecuteAsync(async () =>
        {
            var pendingMigrations = await dbContext.Database.GetPendingMigrationsAsync(ct);

            if (pendingMigrations.Any())
            {
                _logger.LogInformation("Applying {Count} pending migrations...",
                    pendingMigrations.Count());

                await dbContext.Database.MigrateAsync(ct);

                _logger.LogInformation("Migrations applied successfully.");
            }
            else
            {
                _logger.LogInformation("No pending migrations. Database is up to date.");
            }
        });
    }
}

AppHost 配置

var builder = DistributedApplication.CreateBuilder(args);

var postgres = builder.AddPostgres("postgres");
var db = postgres.AddDatabase("appdb");

// Migrations run first, then exit
var migrations = builder.AddProject<Projects.MyApp_MigrationService>("migrations")
    .WaitFor(db)
    .WithReference(db);

// API waits for migrations to complete
var api = builder.AddProject<Projects.MyApp_Api>("api")
    .WaitForCompletion(migrations)  // Key: waits for migrations to finish
    .WithReference(db);

---

模式 4：使用 ExecutionStrategy 处理瞬时故障

对于可能发生瞬时故障的操作，始终使用 CreateExecutionStrategy()：

public async Task UpdateWithRetryAsync(Guid id, Action<Order> update)
{
    var strategy = _dbContext.Database.CreateExecutionStrategy();

    await strategy.ExecuteAsync(async () =>
    {
        var order = await _dbContext.Orders
            .AsTracking()
            .FirstOrDefaultAsync(o => o.Id == id);

        if (order is null) return;

        update(order);
        await _dbContext.SaveChangesAsync();
    });
}

重要： 不能将 CreateExecutionStrategy() 与用户发起的事务一起使用。如果需要在事务中重试：

var strategy = _dbContext.Database.CreateExecutionStrategy();

await strategy.ExecuteAsync(async () =>
{
    // Transaction must be INSIDE the strategy callback
    await using var transaction = await _dbContext.Database.BeginTransactionAsync();

    try
    {
        // ... your operations ...
        await _dbContext.SaveChangesAsync();
        await transaction.CommitAsync();
    }
    catch
    {
        await transaction.RollbackAsync();
        throw;
    }
});

---

模式 5：使用 ExecuteUpdate/ExecuteDelete 执行批量操作

对于批量操作，请使用 EF Core 7+ 的 ExecuteUpdateAsync 和 ExecuteDeleteAsync，而不是加载实体：

// ❌ SLOW - Loads all entities into memory
var expiredOrders = await _db.Orders
    .Where(o => o.ExpiresAt < DateTimeOffset.UtcNow)
    .ToListAsync();

foreach (var order in expiredOrders)
{
    order.Status = OrderStatus.Expired;
}
await _db.SaveChangesAsync();

// ✅ FAST - Single SQL UPDATE statement
await _db.Orders
    .Where(o => o.ExpiresAt < DateTimeOffset.UtcNow)
    .ExecuteUpdateAsync(setters => setters
        .SetProperty(o => o.Status, OrderStatus.Expired)
        .SetProperty(o => o.UpdatedAt, DateTimeOffset.UtcNow));

// ✅ FAST - Single SQL DELETE statement
await _db.Orders
    .Where(o => o.Status == OrderStatus.Cancelled && o.CreatedAt < cutoffDate)
    .ExecuteDeleteAsync();

---

常见问题

1. 使用 NoTracking 时忘记更新

// ❌ Silent failure - entity not tracked
var customer = await _db.Customers.FindAsync(id);
customer.Name = "New Name";
await _db.SaveChangesAsync(); // Does nothing!

// ✅ Explicit update
var customer = await _db.Customers.FindAsync(id);
customer.Name = "New Name";
_db.Customers.Update(customer);
await _db.SaveChangesAsync();

2. N+1 查询问题

// ❌ N+1 queries - one query per order
var customers = await _db.Customers.ToListAsync();
foreach (var customer in customers)
{
    var orders = customer.Orders; // Lazy load triggers query
}

// ✅ Eager loading - single query
var customers = await _db.Customers
    .Include(c => c.Orders)
    .ToListAsync();

3. 多个 DbContext 实例导致跟踪冲突

// ❌ Tracking conflict - entity tracked by different context
var order1 = await _db1.Orders.AsTracking().FindAsync(id);
var order2 = await _db2.Orders.AsTracking().FindAsync(id);
order2.Status = OrderStatus.Shipped;
await _db2.SaveChangesAsync(); // May throw or behave unexpectedly

// ✅ Use single context or detach first
_db1.Entry(order1).State = EntityState.Detached;

4. 未始终一致地使用异步操作

// ❌ Blocking call in async context
var orders = _db.Orders.ToList(); // Blocks thread

// ✅ Async all the way
var orders = await _db.Orders.ToListAsync();

5. 在循环中查询

// ❌ Query per iteration
foreach (var orderId in orderIds)
{
    var order = await _db.Orders.FindAsync(orderId);
    // process order
}

// ✅ Single query
var orders = await _db.Orders
    .Where(o => orderIds.Contains(o.Id))
    .ToListAsync();

---

DI 中的 DbContext 生命周期

ASP.NET Core（默认采用 Scoped 生命周期）

// Scoped = one instance per HTTP request
builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseNpgsql(connectionString));

后台服务（创建作用域）

public class MyBackgroundService : BackgroundService
{
    private readonly IServiceProvider _serviceProvider;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // ✅ Create scope for each unit of work
        using var scope = _serviceProvider.CreateScope();
        var dbContext = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();

        // ... use dbContext ...
    }
}

Actor / 长生命周期对象（工厂模式）

public class OrderActor : ReceiveActor
{
    private readonly IDbContextFactory<ApplicationDbContext> _dbFactory;

    public OrderActor(IDbContextFactory<ApplicationDbContext> dbFactory)
    {
        _dbFactory = dbFactory;

        ReceiveAsync<GetOrder>(async msg =>
        {
            // Create fresh context for each operation
            await using var db = await _dbFactory.CreateDbContextAsync();
            var order = await db.Orders.FindAsync(msg.OrderId);
            Sender.Tell(order);
        });
    }
}

// Registration
builder.Services.AddDbContextFactory<ApplicationDbContext>(options =>
    options.UseNpgsql(connectionString));

---

模式 6：使用查询拆分避免笛卡尔积爆炸

通过 Include() 加载多个导航集合时，EF Core 会生成单个查询，可能导致笛卡尔积爆炸。如果有 10 个订单，每个订单有 10 个商品，结果会是 100 行，而不是 10 + 10 行。

全局配置（适用于大多数情况）

在 DbContext 配置中全局启用查询拆分：

services.AddDbContext<ApplicationDbContext>(options =>
    options.UseNpgsql(connectionString, npgsqlOptions =>
        {
            npgsqlOptions.UseQuerySplittingBehavior(QuerySplittingBehavior.SplitQuery);
        }));

单个查询覆盖设置

确定单个查询效率更高时，使用单查询：

// Use single query when you know the structure is well-understood
var orders = await dbContext.Orders
    .Include(o => o.Items)
    .Include(o => o.Payments)
    .AsSingleQuery()  // Override global split behavior
    .ToListAsync();

权衡

行为	优点	缺点
SplitQuery	不会产生笛卡尔积爆炸，更适合大型集合	需要多次往返，可能存在一致性问题
SingleQuery	只需一次往返，具备事务一致性	多个集合可能导致笛卡尔积爆炸

建议：全局默认使用 SplitQuery，对于已知单查询效果更好的特定查询，使用 AsSingleQuery() 覆盖设置。

适合优先使用 SingleQuery 的情况

• 小型且结构明确的导航关系图（2-3 层）
• 总是需要所有相关数据的查询
• 往返开销低于笛卡尔积爆炸影响的性能关键路径

适合优先使用 SplitQuery 的情况

• 大型或难以预测的导航关系图
• 多对多关系
• 加载的集合可能并非全部都需要的查询

---

使用 EF Core 进行测试

内存中提供程序（仅用于单元测试）

// Only for simple unit tests - doesn't match real database behavior
var options = new DbContextOptionsBuilder<ApplicationDbContext>()
    .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString())
    .Options;

using var context = new ApplicationDbContext(options);

使用 TestContainers 的真实数据库（集成测试）

有关正确的数据库测试方式，请参阅 testcontainers-integration-tests 技能。

// Use real PostgreSQL in container
var container = new PostgreSqlBuilder()
    .WithImage("postgres:16-alpine")
    .Build();

await container.StartAsync();

var options = new DbContextOptionsBuilder<ApplicationDbContext>()
    .UseNpgsql(container.GetConnectionString())
    .Options;