String ID -> schema.rb incorrect?


I made one of my ID columns a string, because I don't want my IDs to
be guessable (I use a md5 hash of some system parameters with
rehashing in case of collisions.)

My migration reads:
change_column :users, :id, :string
change_column :comments, :user_id, :string

Afterwards executing this my db/schema.rb looks like this:
create_table "comments", :force => true do |t|
    t.integer "commentable_id"
    t.string "commentable_type"
    t.string "user_id"
    t.datetime "date"
    t.text "body"
#so far so good
create_table "users", :force => true do |t|
    t.string "email"
    t.string "pwd_hash"
    t.boolean "is_active"
    t.datetime "last_activity"
    t.string "pwd_salt"
    t.string "activation_key"
    t.datetime "signup_date"

hmm, I consider it a bug, because running the migrations and loading
the schema from schema.rb lead to different schemas.
What do you think?