Laravel Eloquent Relationships 提供了五種主要類型：1. 一對(duì)一使用 hasOne 和 belongsTo；2. 一對(duì)多使用 hasMany 和 belongsTo；3. 多對(duì)多使用 belongsToMany 并創(chuàng)建中間表；4. 間接關(guān)聯(lián)使用 hasManyThrough；5. 多態(tài)關(guān)聯(lián)使用 morphTo 和 morphMany，每種關(guān)系通過(guò)在模型中定義方法實(shí)現(xiàn)，Eloquent 自動(dòng)處理底層查詢，使數(shù)據(jù)訪問(wèn)更直觀高效。

Laravel Eloquent Relationships: A Practical Guide

Laravel’s Eloquent ORM makes working with database relationships in PHP both intuitive and powerful. If you're building a Laravel app and need to connect models — like users to posts, orders to products, or categories to articles — Eloquent relationships are the way to go. This guide breaks down the most common relationship types with clear examples you can use right away.

1. One-to-One Relationship

Use this when one record in a table is linked to exactly one record in another table.

Example: A User has one Profile.

Database Setup

// users table
Schema::create('users', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->string('email');
    $table->timestamps();
});

// profiles table
Schema::create('profiles', function (Blueprint $table) {
    $table->id();
    $table->foreignId('user_id')->constrained()->onDelete('cascade');
    $table->string('phone')->nullable();
    $table->text('bio')->nullable();
    $table->timestamps();
});

Model Definition

// User.php
public function profile()
{
    return $this->hasOne(Profile::class);
}

// Profile.php
public function user()
{
    return $this->belongsTo(User::class);
}

Usage

$user = User::find(1);
echo $user->profile->bio;

// Create profile for user
$user->profile()->create([
    'phone' => '123-456-7890',
    'bio' => 'Laravel developer'
]);

Note: hasOne expects the foreign key on the related model (profiles.user_id).

2. One-to-Many Relationship

Used when a single record can have multiple related records.

Example: A User has many Posts.

Database Setup

// posts table
Schema::create('posts', function (Blueprint $table) {
    $table->id();
    $table->foreignId('user_id')->constrained();
    $table->string('title');
    $table->text('body');
    $table->timestamps();
});

Model Definition

// User.php
public function posts()
{
    return $this->hasMany(Post::class);
}

// Post.php
public function user()
{
    return $this->belongsTo(User::class);
}

Usage

$user = User::find(1);
foreach ($user->posts as $post) {
    echo $post->title;
}

// Create a post
$user->posts()->create([
    'title' => 'My First Post',
    'body' => 'Hello World'
]);

3. Many-to-Many Relationship

Used when both sides can have multiple records from the other side. Requires a pivot table.

Example: User belongs to many Roles, and a Role can belong to many Users.

Database Setup

// roles table
Schema::create('roles', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->timestamps();
});

// role_user pivot table (or use roles_users)
Schema::create('role_user', function (Blueprint $table) {
    $table->foreignId('user_id')->constrained()->onDelete('cascade');
    $table->foreignId('role_id')->constrained()->onDelete('cascade');
    $table->primary(['user_id', 'role_id']);
});

Model Definition

// User.php
public function roles()
{
    return $this->belongsToMany(Role::class);
}

// Role.php
public function users()
{
    return $this->belongsToMany(User::class);
}

Usage

$user = User::find(1);
foreach ($user->roles as $role) {
    echo $role->name;
}

// Attach a role
$user->roles()->attach($roleId);

// Sync roles (replace all)
$user->roles()->sync([1, 2, 3]);

// With pivot data (e.g., timestamps or extra fields)
$user->roles()->attach($roleId, ['created_at' => now()]);

Tip: Laravel automatically infers the pivot table name from the singular models in alphabetical order (role_user). You can override it by passing a second argument:
belongsToMany(Role::class, 'user_roles')

4. Has Many Through

Use when you want to access distant relations via an intermediate model.

Example: A Country has many Posts through its Users.

Model Setup

// Country.php
public function posts()
{
    return $this->hasManyThrough(Post::class, User::class);
}

Assumes:

• countries → users (country_id) → posts (user_id)

Usage

$country = Country::find(1);
foreach ($country->posts as $post) {
    echo $post->title;
}

5. Polymorphic Relationships

Use when a model can belong to more than one other model on a single association.

Example: Both Post and User can have Images.

Database Setup

// images table
Schema::create('images', function (Blueprint $table) {
    $table->id();
    $table->string('url');
    $table->morphs('imageable'); // Creates imageable_type and imageable_id
    $table->timestamps();
});

Model Definition

// Image.php
public function imageable()
{
    return $this->morphTo();
}

// Post.php
public function images()
{
    return $this->morphMany(Image::class, 'imageable');
}

// User.php
public function images()
{
    return $this->morphMany(Image::class, 'imageable');
}

Usage

$post = Post::find(1);
$image = $post->images()->create(['url' => 'photo.jpg']);

// Get the owner of the image
$image = Image::find(1);
$owner = $image->imageable; // Returns Post or User instance

Bonus: Defining Custom Foreign Keys

Sometimes your column names don’t follow Laravel’s convention.

// Custom foreign key
return $this->belongsTo(User::class, 'author_id');

// Custom primary key on related model
return $this->belongsTo(User::class, 'author_id', 'uuid');

Summary of Relationship Methods

Eloquent relationships make your Laravel code cleaner and more expressive. Start with the basics — hasOne, hasMany, and belongsTo — then expand to belongsToMany and polymorphic types as your app grows.

Basically, define the relationship once in your model, and Eloquent handles the joins and queries for you.

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