Laravel transform not working as intended on collection

I have a collection which is an eloquent query. There is one column where I want to replace the value with another value. I am using the transform function to do this however it is not working as intended.

Here is my query in the controller

$articles = KnowledgeBaseArticle::getArticlesByDepartment($department)
    ->get()
    ->transform(function ($article) {
      $article->category_id = KnowledgeBaseCategory::find($article->category_id)->name;
    });

and the getArticlesByDepartment query from the model:

public function scopeGetArticlesByDepartment($query, $department){
            return $query->where('department', $department)
                        ->select('title', 'updated_at', 'department', 'id', 'category_id')
                        ->orderBy('title', 'asc');
    }

I want to return it so that all the rows with column category_id is replaced with the category name. You can see I am trying to do this by using $article->category_id by using find on the KnowledgeBaseCategory model to retrieve this. However this is not working at all and when I die and dump, I get an single column array full of nulls.

When I have died and dumped $article->category_id & find query inside the transform, it is returning the correct category name, it is just not replacing the category_id column with the category name.

I have also tried map instead of transform and got the same result.

If it matters, I am later on converting this data into JSON.

Where am I going wrong? Thanks

via Chebli Mohamed