Agent skill

dotnet-entity-framework-core

Design, tune, or review EF Core data access with proper modeling, migrations, query translation, performance, and lifetime management for modern .NET applications.

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SKILL.md

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

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

csharp
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

csharp
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

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

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

Project to DTOs

csharp
// 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

csharp
// 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)

csharp
// 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

bash
# 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

csharp
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

Anti-Pattern Why It's Bad Better Approach
ToList() then filter Loads all data to memory Filter in query
Multiple DbContext per request Transaction issues Scoped lifetime
Lazy loading everywhere N+1 queries Explicit Include
Generic repository wrapper Removes query power Use DbContext directly
Ignoring generated SQL Hidden performance issues Log and review
SaveChanges() in loops Many roundtrips Batch then save

Performance Best Practices

  1. Index frequently queried columns:

    csharp
    builder.HasIndex(p => p.CreatedAt);
builder.HasIndex(p => new { p.Category, p.Status });

  2. Use pagination:

    csharp
    var page = await db.Products
    .OrderBy(p => p.Id)
    .Skip(pageSize * pageNumber)
    .Take(pageSize)
    .ToListAsync();

  3. Batch updates (EF Core 7+):

    csharp
    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:

    csharp
    // 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

csharp
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

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