Why does scaffold ignores some fields?


somehow the 'scaffold' method does not emits HTML fields for all the
of my models. For example, this simple controller:

  class CatalogController < ApplicationController
    scaffold :fact

and a model backed by (sqlite3, tested also with mysql)

  create table "facts" (
    "id" integer primary key autoincrement,
    "type" varchar not null,
    "device_id" integer not null,
    "stated_on" date not null,
    "created_at" datetime not null,
    "updated_at" datetime not null,
    "approved" boolean default false

produces HTML fields only for "Stated on", "Created at", "Updated at"
and "Approved". "type" and "device_id" are ignored. Why?