Getting Array of Related IDs from a belongsToMany Relation in Laravel 5.4

Dealing with framework updates, especially when moving between minor versions like Laravel 5.3 and 5.4, often reveals subtle but significant changes in Eloquent methods. As a senior developer, I frequently encounter situations where older patterns break, forcing us to adapt to the newer, more idiomatic ways of handling data retrieval.

I recently encountered this exact scenario while working with a belongsToMany relationship: attempting to retrieve an array of related IDs from a pivot relationship in Laravel 5.4 resulted in an error. This post will walk through why the previous approach failed and demonstrate the modern, robust alternatives for fetching related IDs efficiently using Eloquent.

The Problem: Breaking Change in Eloquent

The original approach that worked fine in older versions relied on a method that has since been removed or deprecated:

$procedure = Procedure::findOrFail($id);
// This caused an error in Laravel 5.4+
$attached_stages = $procedure->stages()->getRelatedIds()->toArray();

As you observed, calling $relation->getRelatedIds() resulted in a fatal error because this method no longer exists on the Query Builder or the relationship itself in recent Laravel versions. This is a common theme in framework evolution—methods are streamlined or replaced to improve performance and consistency, often pushing developers toward explicit eager loading rather than relying on shortcut methods.

The Solution: Modern Eloquent Approaches

The key to solving this is moving away from specialized, potentially deprecated methods and instead leveraging core Eloquent capabilities for querying the pivot data directly. There are several clean ways to achieve the goal of getting an array of related IDs.

Method 1: Using with() and pluck() (The Recommended Way)

If your primary goal is to load the relationship and extract only the foreign keys you need, using eager loading combined with the pluck() method on the relationship is the most efficient approach. This tells Eloquent exactly what data it needs to fetch from the pivot table.

Assuming your Procedure model has a stages() relationship defined as:

public function stages()
{
    return $this->belongsToMany(Stage, 'procedure_stage', 'procedure_id', 'stage_id')
        ->withPivot('id', 'status')
        ->withTimestamps();
}

You can retrieve the related IDs like this:

$procedure = Procedure::findOrFail($id);

// Load the relationship and pluck only the 'id' column from the pivot table
$relatedStageIds = $procedure->stages()->pluck('id')->toArray();

This method is highly efficient because it delegates the filtering and extraction directly to the underlying database query, avoiding fetching unnecessary model hydration. This principle of optimizing data retrieval is central to building performant applications, much like how you should approach complex database interactions when working with Eloquent structures on platforms like https://laravelcompany.com.

Method 2: Direct Pivot Table Query (For Raw Performance)

If you need maximum raw performance and are only concerned with the IDs and don't need to hydrate the full Stage models, you can bypass the Eloquent relationship loading entirely and query the pivot table directly using the whereHas() or direct join methods.

To get the IDs related to a specific procedure ID:

$procedureId = $id;

$relatedIds = DB::table('procedure_stage')
    ->where('procedure_id', $procedureId)
    ->pluck('stage_id')
    ->toArray();

This approach is excellent when dealing with very large datasets where the overhead of loading Eloquent relationships might be unnecessary. It gives you direct control over the SQL execution, which is invaluable for optimizing high-volume operations.

Conclusion

The shift in how Laravel handles relationship data reflects a move towards explicit and decoupled querying. Instead of relying on specialized methods that change with framework updates (like the defunct getRelatedIds()), we should favor standard Eloquent tools like with() combined with pluck() or direct database queries when fetching related IDs. By adopting these patterns, you ensure your code remains resilient across framework upgrades and maintains optimal performance, leading to cleaner, more maintainable application logic.