NULL datetime field inserted when value is assigned to model object

This is pretty baffling. I have the following model object:

create_table "activities", :force => true do |t|
    t.integer "user_id", :limit => 11
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer "location_id", :limit => 11
    t.datetime "


class Activity < ActiveRecord::Base

  belongs_to :User
  belongs_to :Location

  attr_accessor :event_date
  attr_accessor :user
  attr_accessor :location


my activities_controller.rb:

    activity =[:activity])

    activity.event_date =

    activity.user_id =

It throws an error b/c it's trying to insert NULL for event_date in
the MYSQL database:
ActiveRecord::StatementInvalid (Mysql::Error: Column 'event_date'
cannot be null: INSERT INTO `activities` (`update\
d_at`, `location_id`, `user_id`, `event_date`, `created_at`)
VALUES('2008-07-30 05:16:00', 1, 1, NULL, '2008-07-30 \

I have no idea why. If I explicitly print out
'activity.event_date.to_s' is gives me the date of today.

If I remove the stipulation the "event_date" can be NULL it just
insert a NULL value.

thanks in advance for help

in a controller I create an "Activiti