EF Core Fluent API — Advanced Configuration

Introduction

When Data Annotations aren't enough, the Fluent API in OnModelCreating or IEntityTypeConfiguration<T> gives ShopNest Banking precise control over columns, indexes, concurrency, and value objects.

After this article you will

• Separate configuration with IEntityTypeConfiguration
• Map columns, schemas, indexes, defaults
• Configure owned entities and shadow properties
• Implement optimistic concurrency tokens
• Understand TPH inheritance mapping

Prerequisites

• Article 17 — EF Core Relationships
• ShopNest DbContext with Products/Orders from Articles 13–15
• SQL Server or LocalDB configured

Concept deep-dive

Fluent API vs Annotations: Both apply — Fluent wins on conflicts. Use Fluent for team-wide DB rules; Annotations for simple validation on POCOs.

public class AccountConfiguration : IEntityTypeConfiguration<Account>
{
    public void Configure(EntityTypeBuilder<Account> b)
    {
        b.ToTable("Accounts", schema: "banking");
        b.HasIndex(a => a.AccountNumber).IsUnique();
        b.Property(a => a.Balance).HasPrecision(18, 2);
        b.Property(a => a.RowVersion).IsRowVersion(); // concurrency
        b.OwnsOne(a => a.Address, addr => {
            addr.Property(x => x.City).HasMaxLength(80);
        });
    }
}

// DbContext
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.ApplyConfigurationsFromAssembly(typeof(ShopNestDbContext).Assembly);
}

Shadow properties: b.Property<DateTime>("LastModified"); — not on C# class but in DB.

TPH: Discriminator column for Account types (Savings/Current) in one table.

Hands-on — ShopNest Banking Application

1. Account, Transaction entities with precision and unique AccountNumber index.
2. AccountConfiguration class in Configurations/ folder.
3. Test concurrency: two updates — DbUpdateConcurrencyException.
4. Owned Address value object on Customer.

Common errors & best practices

• Precision missing on decimal — money rounding bugs in SQL.
• All config in one giant OnModelCreating — split into IEntityTypeConfiguration files.

Interview questions

Q: IEntityTypeConfiguration benefit?
A: One class per entity — clean, testable, discoverable via ApplyConfigurationsFromAssembly.

Q: Optimistic concurrency?
A: RowVersion/timestamp — SaveChanges fails if row changed since read; UI shows refresh message.

Summary

• Fluent API is the source of truth for complex mappings
• IEntityTypeConfiguration keeps DbContext thin
• Indexes and precision matter for banking-grade data
• Owned types model value objects without extra tables

Previous: EF Core Relationships
Next: EF Core Repository Pattern and Unit of Work

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