Element API - Entries Related to a Category

Question

In Craft 3, I want to list all the categories in a group and all the entries related to them. How can I do this?

Answer 1

The slower but easier approach is to fetch all of them and search for related entries

'elementType' => Category::class,
'criteria'    => [
    'group'   => 'yourGroup', 
],
'paginate'    => false,
'transformer' => function(Category $category) {
     $entries = Entry::find()
        ->relatedTo($category)
        ->select(['title'])
        ->asArray()
        ->column();

    return [
        'title'   => $category->title,
        'entries' => $entries     
    ];
}

Learn more about Element Queries especially the different ways to fetch entries via one(), all(), column()