Rails Model Constants

I have Attachments and Category models so when a user uploads a file, they can select a category for that attachment. I want the categories to be static for now. Advice on how to create static Category model options? I have this right now but I get the following error: undefined method 'title' for Syllabus":String

Category Model

class Category < ActiveRecord::Base
  CATEGORY = ['Syllabus', 'Assignments', 'Handouts', 'Lectures', 'Other']
  has_many :attachments
end

Attachment new.html.erb

<%= simple_form_for([@group, @group.attachments.build]) do |f| %>

  <%= f.collection_select :category_id, Category::CATEGORY, :id, :title, { promt: "Choose a Category" } %>

<%= f.submit %>

<% end %>

Attachment Model

class Attachment < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
  belongs_to :category
end

Schema

 create_table "categories", force: :cascade do |t|
    t.string   "title"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

 create_table "attachments", force: :cascade do |t|
    t.string   "title"
    t.datetime "created_at",  null: false
    t.datetime "updated_at",  null: false
    t.integer  "user_id"
    t.string   "name"
    t.integer  "group_id"
    t.integer  "category_id"
 end