How to get automatic id column name for polymorphic manyToMany relationship subquerying? – Laravel

The below function allows me to retrieve related entries of a given model type that have, for instance, overlapping tags associated with them.

<?php

public function getRelated($entity, $fields = [], $relations = []) {
    $related = new EloquentCollection();

    // get models of same entity type that have relations in common

    if (count($relations)) {
        foreach ($relations as $relation) {
            // check if the current model in use has any of the currently iterated relationship at all
            if ($entity->{$relation}()->exists()) {
                // if yes, get all the ids of the related models, for instance tags
                $relationIds = $entity->{$relation}()->get()->pluck('id')->toArray();

                // now, get all same type models except the current one
                $relationEntities = $this->entity::whereNot('id', $entity->id)
                    // also except the ones we already found
                    ->whereNotIn('id', $related->modelKeys())
                    // and only get those which also have current relationship (e.g. tags)
                    ->whereHas($relation, function (Builder $query) use($relationIds, $relation) {
                        $query->whereIn($relation . '.id', $relationIds);
                    })
                    ->get()
                    ->sortKeysDesc()
                    ->take(10);

                $related = $related->concat($relationEntities->except($related->modelKeys()));
            }
        }
    }
}

This works well. So I could load a blog post, for instance, and show related blog posts that have some of the same tags by calling:

$post = Post::find(1);
$related = $this->getRelated($post, [], ['tags']);

Now, I also need to respect other types than tags, hence $relations is an array: "Show me related blog posts that have any of these tags, categories, you name it, in common."

My first problem was in the subquery. I had to hardcode the id in the relation table:

$query->whereIn('id', $relationIds);
had to be
$query->whereIn($relation . '.id', $relationIds);

So now my problem is that $relation . '.id' doesnt work for all cases. The relationship fooBars() would expect foo_bar.id but looks for fooBar.id.

Is there no automatic way of Laravel automatically knowing the column name based on the relationship I am currently querying? If not, I must, for each relatioship I am querying, define the column name (of the id) of the relationship table. Is there something I am missing?

Or can I somehow load the relationship and look at the DB scheme? Whats best practice here?

One way could be to use snake case ($query->whereIn(Str::of($relation)->snake() . '.id', $relationIds);) but that feels vague and dirty.

All of these are polymorphic morphToMany/ morphedByMany.

Edit: For better understanding I’m adding an example of what my models look like. They all use default naming conventions. Again, I want to get related Posts that have either some Tags or FooBars in common.

<?php

class Post extends Entity
{
    public function tags()
    {
        return $this->morphToMany(Tag::class, 'taggable');
    }

    public function fooBars()
    {
        return $this->morphToMany(FooBar::class, 'foo_barable');
    }    
}

class Tag extends Entity
{
    public function posts()
    {
        return $this->morphedByMany(Post::class, 'taggable');
    }
}

class FooBar extends Entity
{
    public function posts()
    {
        return $this->morphedByMany(Post::class, 'foo_barable');
    }
}