A code-first approach starts with application entities rather than an existing database schema. Entity Framework Core (EF Core) uses those definitions and its configuration to generate tables; for a persistent database, it can generate migrations to create the schema. Database-first mapping starts with an existing schema instead. section 5.1
A minimal EF Core model has three parts:
- An entity class defines the data to store. A Book with Id and Title maps to a Books table; EF Core treats Id as the primary key by convention. A different property can be marked with [Key]. section 5.2
- A class derived from DbContext identifies the model. Its DbSet<Book> Books property includes Book in that model, while OnConfiguring selects a database provider. section 5.2
- Application code adds entities, calls SaveChanges, and queries the DbSet. Do not use one DbContext concurrently across parallel tasks or threads. section 5.2
Listing 5.3 A DbContext that holds the Book class
using Microsoft.EntityFrameworkCore;
namespace ManningBooks;
public class CatalogContext : DbContext #1
{
public DbSet<Book> Books { get; set; } #2
protected override void OnConfiguring(DbContextOptionsBuilder options)
=> options.UseInMemoryDatabase("ManningBooks"); #3
}
The context listing connects the Book entity to a DbSet and configures an in-memory provider. That provider keeps the first application runnable without a separate database, but is intended only for development. section 5.2
Relationships can also begin in entity code. A Book collection of Ratings is recognized as one-to-many; EF Core gives Ratings a foreign key even without an explicit BookId property. Collections on both Book and Tag express a many-to-many relationship, for which EF Core creates a join table. Explicit foreign keys are possible, but these examples rely on conventions. section 5.2.3
Add ratings through an entity
Create a Book, add two Rating objects to its Ratings collection, add the Book to the context, and call SaveChanges. Query the book and use Include(b => b.Ratings) when printing its ratings. This exercises both the code-defined relationship and the saved data. section 5.2.3
Conventions are a starting point, not a guarantee of every database constraint. In the one-to-one example, database-level enforcement needs additional techniques such as a unique constraint on the foreign key; EF Core does not add those techniques for this code-first schema. section 5.2.3
