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);
    }
}

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));

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

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

// 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();

// 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();

// 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);

# 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

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");
    }
}

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...
}