Rails未将模型字段添加到插入查询

请求参数中有正确的数据,但生成的插入查询每次都有一个空的列。

以下是请求参数:

{"commit"=>"Create",
  "assignment"=>{"start_date"=>"08/04/2010",
  "project_id"=>"5",
  "allotment"=>"50",
  "person_id"=>"33",
  "end_date"=>"08/05/2010"},
  "authenticity_token"=>"8f157a2a220caf716607162ce9557ab6505aab1a"}

Mysql::Error: Column 'end_date' cannot be null: 
INSERT INTO `assignments` (`start_date`, `created_at`, `project_id`, `updated_at`, `allotment`, `person_id`, `end_date`)
VALUES('2010-08-04', '2010-08-04 03:36:21', 5, '2010-08-04 03:36:21', 50, 33, NULL)

class ChangeDates < ActiveRecord::Migration
  def self.up
    add_column :assignments, :end_date, :date, :null => false
    rename_column :assignments, :date, :start_date
  end

  def self.down
    remove_column :assignments, :end_date
    rename_column :assignments, :start_date, :date
  end
end

我不知道出了什么问题。我已经回滚并迁移了几次更改,但都不走运。我被难住了。