Return only some resources from 2 tables


I'm having trouble selecting only some specific resources from 2 tables.
That's my code:
categories = %w{cat1 cat2 cat3}
categories.each { |category|
  cat = CharacterAttributeCategory.find_by_name category
  @character_attribute_types +=

I tried << before but that doesn't work as well. I get following error:
NoMethodError in CharactersController#new

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+

Any hints?

Do you mean this?

@character_attribute_types = []
@character_attribute_types << CharacterAttributeType.find_all_by_character_attribute_category_id