EF Core Relationships & Migrations

using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

// === ONE-TO-MANY: one Author has many Books ===

// The "one" side. It carries a COLLECTION navigation: List<Book>.
public class Author
{
    public int Id { get; set; }                       // primary key (by convention)
    public string Name { get; set; } = "";
    public List<Book> Books { get; set; } = new();    // navigation: the many side
}

// The "many" side. It carries the FOREIGN KEY + a reference navigation.
p
...

using System;
using System.Collections.Generic;

// 🎯 YOUR TURN — model a one-to-many the same shape EF uses,
//                but as plain in-memory objects so it RUNS here.

class Book
{
    public string Title { get; set; } = "";
}

class Author
{
    public string Name { get; set; } = "";

    // 1) An Author has MANY Books. Give it a collection navigation.
    public List<Book> Books { get; set; } = ___;   // 👉 a new empty list: new()
}

class Program
{
    static void Main()
    {
     
...

using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

// === MANY-TO-MANY: a Book has many Tags, a Tag labels many Books ===
// EF Core 5+ creates the join table for you (a "skip navigation").

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public List<Tag> Tags { get; set; } = new();   // collection on BOTH sides
}

public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public List<
...

using Microsoft.EntityFrameworkCore;

// === ONE-TO-ONE: each Author has exactly one Profile ===
public class Author
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public Profile Profile { get; set; } = null!;   // reference navigation, ONE
}

public class Profile
{
    public int Id { get; set; }
    public string Bio { get; set; } = "";
    public int AuthorId { get; set; }               // FK that is also UNIQUE
    public Author Author { get; set; } = null!;
...

using System.Linq;
using Microsoft.EntityFrameworkCore;

// Three ways to pull related data. Pick deliberately — it decides
// how many round-trips hit the database.

// 1) EAGER loading with Include — fetch related data in the SAME query.
var authors = db.Authors
    .Include(a => a.Books)            // bring each author's Books along
        .ThenInclude(b => b.Tags)    // ...and each book's Tags too (multi-level)
    .ToList();
// ✅ One query (a JOIN) returns authors + books + tags together.

...

using System;
using System.Collections.Generic;
using System.Linq;

// 🎯 YOUR TURN — this is the LINQ you'd run AFTER an Include loads
//                the data. Here the "related data" is plain in-memory
//                lists, so it RUNS and prints.

class Book   { public string Title = ""; public int AuthorId; }
class Author { public int Id; public string Name = ""; }

class Program
{
    static void Main()
    {
        var authors = new List<Author>
        {
            new Author { Id 
...

// === EF Core MIGRATIONS: version control for your database schema ===
// Each migration is like a git commit: it can be applied (Up) or
// reverted (Down). You run these in the terminal or the Package
// Manager Console — NOT in C# code.

// ---- Package Manager Console (Visual Studio) ----
// Add-Migration InitialCreate     // generate a migration from model changes
// Update-Database                 // apply pending migrations to the DB

// ---- dotnet CLI (cross-platform) ----
// dotnet ef 
...