dotnet-entity-framework-core（dotnet 实体框架核心）

Entity Framework Core

Trigger On

• working on DbContext, migrations, model configuration, or EF queries
• reviewing tracking, loading, performance, or transaction behavior
• porting data access from EF6 or custom repositories to EF Core
• optimizing slow database queries

Documentation

• EF Core Overview
• Performance
• Efficient Querying
• Migrations
• What's New in EF Core 9

References

• patterns.md - Query patterns, tracking strategies, loading strategies, projections, compiled queries, pagination, and temporal tables
• anti-patterns.md - Common EF Core mistakes including N+1 queries, large contexts, generic repositories, and missing indexes

Workflow

1. Prefer EF Core for new development unless a documented gap requires Dapper or raw SQL
2. Keep DbContext lifetime scoped — align with unit of work
3. Review query translation — check generated SQL, avoid N+1
4. Treat migrations as first-class — reviewable, not throwaway
5. Be deliberate about provider behavior — cross-provider but not identical
6. Validate with query inspection — not just in-memory mental model

DbContext Patterns

Basic Configuration

public class AppDbContext : DbContext
{
    public DbSet<Product> Products => Set<Product>();
    public DbSet<Order> Orders => Set<Order>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);
    }
}

// Entity Configuration (Fluent API)
public class ProductConfiguration : IEntityTypeConfiguration<Product>
{
    public void Configure(EntityTypeBuilder<Product> builder)
    {
        builder.HasKey(p => p.Id);
        builder.Property(p => p.Name).HasMaxLength(200).IsRequired();
        builder.HasIndex(p => p.Sku).IsUnique();
        builder.HasMany(p => p.OrderItems).WithOne(oi => oi.Product);
    }
}

Registration with DI

builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(connectionString)
           .EnableSensitiveDataLogging()  // Dev only
           .EnableDetailedErrors());      // Dev only

// Or with pooling (better performance)
builder.Services.AddDbContextPool<AppDbContext>(options =>
    options.UseSqlServer(connectionString));

Query Patterns

Use AsNoTracking for Read-Only

// Bad - tracks entities unnecessarily
var products = await db.Products.ToListAsync();

// Good - no tracking overhead
var products = await db.Products
    .AsNoTracking()
    .ToListAsync();

Project to DTOs

// Bad - loads entire entity graph
var orders = await db.Orders
    .Include(o => o.Items)
    .Include(o => o.Customer)
    .ToListAsync();

// Good - loads only needed data
var orders = await db.Orders
    .Select(o => new OrderDto
    {
        Id = o.Id,
        CustomerName = o.Customer.Name,
        ItemCount = o.Items.Count,
        Total = o.Items.Sum(i => i.Price)
    })
    .ToListAsync();

Avoid N+1 Queries

// Bad - N+1 problem
foreach (var order in orders)
{
    var items = await db.OrderItems
        .Where(i => i.OrderId == order.Id)
        .ToListAsync();
}

// Good - eager loading
var orders = await db.Orders
    .Include(o => o.Items)
    .ToListAsync();

// Good - split query for large graphs
var orders = await db.Orders
    .Include(o => o.Items)
    .AsSplitQuery()
    .ToListAsync();

Compiled Queries (EF Core 9)

// Pre-compiled for frequently used queries
private static readonly Func<AppDbContext, int, Task<Product?>> GetProductById =
    EF.CompileAsyncQuery((AppDbContext db, int id) =>
        db.Products.FirstOrDefault(p => p.Id == id));

// Usage
var product = await GetProductById(db, productId);

Migration Patterns

Creating Migrations

# Add migration
dotnet ef migrations add AddProductIndex

# Apply to database
dotnet ef database update

# Generate SQL script
dotnet ef migrations script --idempotent -o migrate.sql

Data Migrations

public partial class AddProductIndex : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.CreateIndex(
            name: "IX_Products_Sku",
            table: "Products",
            column: "Sku",
            unique: true);

        // Data migration (if needed)
        migrationBuilder.Sql(@"
            UPDATE Products
            SET NormalizedName = UPPER(Name)
            WHERE NormalizedName IS NULL");
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.DropIndex(
            name: "IX_Products_Sku",
            table: "Products");
    }
}

Anti-Patterns to Avoid

Performance Best Practices

1. Index frequently queried columns: builder.HasIndex(p => p.CreatedAt); builder.HasIndex(p => new {p.Category, p.Status});
2. Use pagination: var page = await db.Products.OrderBy(p => p.Id).Skip(pageSize * pageNumber).Take(pageSize).ToListAsync();
3. Batch updates (EF Core 7+): await db.Products.Where(p => p.Category == "Obsolete").ExecuteDeleteAsync(); await db.Products.Where(p => p.Category == "Sale").ExecuteUpdateAsync(p => p.SetProperty(x => x.Price, x => x.Price * 0.9m));
4. Minimize network roundtrips: // Bad - 3 roundtrips var product = await db.Products.FindAsync(id); var reviews = await db.Reviews.Where(r => r.ProductId == id).ToListAsync(); var related = await db.Products.Where(p => p.Category == product.Category).ToListAsync(); // Good - 1 roundtrip var data = await db.Products.Where(p => p.Id == id).Select(p => new {Product = p, Reviews = p.Reviews, Related = db.Products.Where(r => r.Category == p.Category).Take(5)}).FirstOrDefaultAsync();

Concurrency Patterns

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

    [ConcurrencyCheck]
    public int Version { get; set; }

    // Or use RowVersion
    [Timestamp]
    public byte[] RowVersion { get; set; }
}

// Handle concurrency conflicts
try
{
    await db.SaveChangesAsync();
}
catch (DbUpdateConcurrencyException ex)
{
    var entry = ex.Entries.Single();
    var databaseValues = await entry.GetDatabaseValuesAsync();
    // Resolve conflict...
}

Deliver

• EF Core models and queries that match the domain
• safer migrations and lifetime management
• performance-aware data access decisions
• proper indexing and query optimization

Validate

• query behavior is intentional (check SQL logs)
• migrations are reviewable and correct
• no N+1 queries in common paths
• indexes exist for filtered/sorted columns
• DbContext lifetime is scoped properly
• concurrency is handled for critical entities