Has-many-through in Laravel - Step-by-Step Execution

Concept Flow - Has-many-through

Start: User Model

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HasManyThrough Relation Defined

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Query: User -> Posts Through Country

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Laravel Builds SQL Join

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Fetch Posts Related to User via Country

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Return Collection of Posts

Shows how Laravel uses a has-many-through relation to get related records through an intermediate model.

Execution Sample

Laravel

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

$posts = User::find(1)->posts;

Defines a has-many-through relation from User to Post through Country, then fetches posts for user with ID 1.

Execution Table

Step	Action	Query Built	Result
1	Find User with ID 1	SELECT * FROM users WHERE id = 1	User model instance with id=1
2	Call posts() relation	Build hasManyThrough SQL join	Prepare SQL to join countries and posts via user
3	Execute SQL to get posts	SELECT posts.* FROM posts INNER JOIN countries ON countries.id = posts.country_id WHERE countries.user_id = 1	Collection of Post models related to User 1 through Country
4	Return posts collection	N/A	Posts related to User 1

💡 All posts related to User 1 through their countries are fetched and returned.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
$user	null	User(id=1)	User(id=1)	User(id=1)
$posts	null	null	Collection of Post models	Collection of Post models

Key Moments - 2 Insights

Why does Laravel join countries and posts tables in the query?

Because has-many-through needs to get posts related to the user through the intermediate country model, so it joins countries and posts tables to find matching posts.

What does the hasManyThrough method parameters mean?

The first parameter is the final related model (Post), the second is the intermediate model (Country) through which the relation passes, as shown in the execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what SQL query is run to fetch posts?

A SELECT * FROM posts WHERE user_id = 1

B SELECT * FROM countries WHERE user_id = 1

C SELECT posts.* FROM posts INNER JOIN countries ON countries.id = posts.country_id WHERE countries.user_id = 1

D SELECT posts.* FROM posts WHERE posts.country_id = 1

💡 Hint

Check step 3 in the execution_table for the exact SQL query.

At which step is the User model instance retrieved?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the action column in execution_table step 1.

If the intermediate model was changed from Country to City, what changes in the execution_table?

A The SQL join would be between cities and posts instead of countries and posts

B The user retrieval query would change

C The posts query would not use any join

D No changes would happen

💡 Hint

Refer to step 3 where the SQL join involves the intermediate model.

Concept Snapshot

Has-many-through relation in Laravel:
- Used to access distant related models through an intermediate model.
- Syntax: hasManyThrough(FinalModel::class, IntermediateModel::class)
- Laravel builds SQL joins automatically.
- Useful for relations like User -> Posts through Country.
- Returns a collection of related models.

Full Transcript

This visual execution shows how Laravel's has-many-through relation works. First, the User model instance is retrieved by ID. Then, calling the posts() relation triggers Laravel to build a SQL join query joining the intermediate Country model and the final Post model. The SQL query fetches posts related to the user through countries. Finally, Laravel returns a collection of Post models. Variables like $user and $posts change from null to model instances as the steps progress. Key points include understanding the SQL join and the meaning of the hasManyThrough parameters. The quizzes test understanding of the SQL query, step order, and effect of changing the intermediate model.