如何在 Laravel 中有效地檢索具有嵌套關係的活動的與會者？

Laravel Nested Relationship Woes: Resolving Complex Data Retrieval

In Laravel, bridging intricate relationships between database tables can often prove challenging. Consider the scenario where you aim to retrieve the attendees of an event by its ID. However, this task is complicated by the presence of intermediary tables within the event-person relationship chain.

To address this issue, it's imperative to meticulously define relationships between your models. Elaborated below are the relevant model relationships:

Event Model:

class Event extends Eloquent
{
    public function city()
    {
        return $this->belongsTo('City');
    }
}

City Model:

class City extends Eloquent
{
    public function companies()
    {
        return $this->hasMany('Company');
    }
}

Company Model:

class Company extends Eloquent
{
    public function persons()
    {
        return $this->hasMany('Person');
    }
}

Person Model:

class Person extends Eloquent
{
    public function eventscore()
    {
        return $this->belongsToMany('Event', 'event_scores', 'person_id', 'event_id')->withPivot('score')->withTimestamps();
    }
}

Having defined these relationships, let's explore two failed attempts at resolving the issue:

return Event::with('city')->with('company')->get();

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

The ultimate solution lies in utilizing eloquent's eager loading capabilities:

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

This query retrieves the event along with its associated city, companies, and the persons associated with those companies.

Alternatively, if only specific fields from the persons table are required:

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

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