```ruby
class CollectionSerializer < ActiveModel::Serializer
  attributes :id, :label, :type

  has_many :children

  def children
    object.children
  end

  def type
    'collection'
  end
end
```

Fix collection api spec - remove used type from collection serializer
user
```ruby
class CollectionSerializer < ActiveModel::Serializer
  attributes :id, :label

  has_many :children

  def children
    object.children
  end

end
```
assistant