Fluent API in EF Core

What is Fluent API?

Fluent API provides a way to configure entity mappings and relationships using method chaining in the OnModelCreating method. It offers more configuration options than Data Annotations.

Basic Configuration

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>(entity =>
    {
        entity.HasKey(e => e.Id);
        entity.Property(e => e.Name).IsRequired().HasMaxLength(200);
        entity.Property(e => e.Price).HasColumnType("decimal(18,2)");
        entity.ToTable("Products");
    });
}

Property Configuration

modelBuilder.Entity<Product>()
    .Property(p => p.Name)
    .IsRequired()
    .HasMaxLength(200)
    .HasColumnName("ProductName")
    .HasDefaultValue("Unknown");

modelBuilder.Entity<Product>()
    .Property(p => p.Price)
    .HasColumnType("decimal(18,2)")
    .HasPrecision(18, 2);

modelBuilder.Entity<Product>()
    .Property(p => p.CreatedAt)
    .HasDefaultValueSql("GETUTCDATE()");

Primary Keys

// Single column primary key
modelBuilder.Entity<Product>()
    .HasKey(p => p.Id);

// Composite primary key
modelBuilder.Entity<OrderItem>()
    .HasKey(oi => new { oi.OrderId, oi.ProductId });

// Alternate key
modelBuilder.Entity<Product>()
    .HasAlternateKey(p => p.SKU);

Indexes

// Single column index
modelBuilder.Entity<Product>()
    .HasIndex(p => p.Name);

// Unique index
modelBuilder.Entity<User>()
    .HasIndex(u => u.Email)
    .IsUnique();

// Composite index
modelBuilder.Entity<Product>()
    .HasIndex(p => new { p.CategoryId, p.IsActive })
    .HasDatabaseName("IX_Product_Category_Active");

// Filtered index
modelBuilder.Entity<Product>()
    .HasIndex(p => p.Price)
    .HasFilter("[IsActive] = 1");

Relationships

// One-to-Many
modelBuilder.Entity<Post>()
    .HasOne(p => p.Blog)
    .WithMany(b => b.Posts)
    .HasForeignKey(p => p.BlogId)
    .OnDelete(DeleteBehavior.Cascade);

// One-to-One
modelBuilder.Entity<User>()
    .HasOne(u => u.Profile)
    .WithOne(p => p.User)
    .HasForeignKey<UserProfile>(p => p.UserId);

// Many-to-Many
modelBuilder.Entity<Student>()
    .HasMany(s => s.Courses)
    .WithMany(c => c.Students)
    .UsingEntity(j => j.ToTable("StudentCourses"));

Table Mapping

// Table name
modelBuilder.Entity<Product>()
    .ToTable("Products", schema: "dbo");

// Table splitting
modelBuilder.Entity<Order>()
    .ToTable("Orders");

modelBuilder.Entity<OrderDetails>()
    .ToTable("Orders");

modelBuilder.Entity<OrderDetails>()
    .HasOne(od => od.Order)
    .WithOne(o => o.Details)
    .HasForeignKey<OrderDetails>(od => od.OrderId);

Value Conversions

modelBuilder.Entity<Product>()
    .Property(p => p.Status)
    .HasConversion<string>();

modelBuilder.Entity<User>()
    .Property(u => u.DateOfBirth)
    .HasConversion(
        v => v.ToUniversalTime(),
        v => DateTime.SpecifyKind(v, DateTimeKind.Utc));

Global Query Filters

modelBuilder.Entity<Product>()
    .HasQueryFilter(p => !p.IsDeleted);

modelBuilder.Entity<Post>()
    .HasQueryFilter(p => p.IsPublished);

Owned Entities

modelBuilder.Entity<Order>().OwnsOne(o => o.ShippingAddress);
modelBuilder.Entity<Order>().OwnsOne(o => o.BillingAddress);

Summary

Fluent API in EF Core provides comprehensive configuration options for entities, properties, relationships, and database mappings through method chaining in OnModelCreating.