How to Handle Nested Relationships in Laravel: Retrieving Persons Associated with an Event?

Laravel Nested Relationships: Breaking Down Complex Database Queries

In Laravel, navigating complex relationships between multiple database tables can be a daunting task. This question exemplifies such a challenge: retrieving persons associated with an event when multiple intermediate tables exist.

Understanding the Database Schema

The database schema consists of four tables: events, cities, companies, and persons. The Event model has a many-to-one relationship with City, and City has a one-to-many relationship with both Company and Event. Company has a one-to-many relationship with Person.

Nested Relationship Query

The user requires a query that retrieves persons subscribed to an event by ID. To achieve this, we need to navigate through the city and company relationships.

Solution

The straightforward solution is to use the with method to eagerly load the nested relationships:

return Event::with('city.companies.persons')->get();

This query retrieves all events, along with their associated cities, companies, and persons.

Field Selection

If you only need specific fields from the persons table, such as ID and firstname, use a closure to modify the query:

return Event::with(['city.companies.persons' => function ($query) {
    $query->select('id', 'firstname');
}])->get();

This modified query retrieves persons with only the specified fields.

Avoiding Eager Loading

If eager loading is not desired, you can use the whereHas method to constrain the query based on the existence of related models:

return Event::whereHas('city.companies.persons', function ($query) {
    $query->where('company_id', $company_id);
})->get();

This query retrieves events that have at least one person associated with a specific company.

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