ruby-on-rails – 导轨中的外键4

class Indicator < ActiveRecord::Base
  belongs_to :objetive
  belongs_to :responsible,class_name: "Person"

end

迁移脚本：

class AddFksToIndicator < ActiveRecord::Migration
  def change
      add_reference :indicators,:objective,index: true
      add_reference :indicators,:responsible,index: true
  end
end

在运行迁移时一切正常,所以我尝试在控制台中：

2.0.0p247 :002 > i = Indicator.new
 => #<Indicator id: nil,name: nil,created_at: nil,updated_at: nil,objective_id: nil,responsible_id: nil> 
2.0.0p247 :002 > i.objective_id = 0
2.0.0p247 :003 > i.save

令我惊讶的是,指标已经保存,并且没有id = 0的目标.

最后,我检查了指标表模式,得到：

CREATE TABLE "indicators" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,"name" varchar(255),"created_at" datetime,"updated_at" datetime,"objective_id" integer,"responsible_id" integer);
CREATE INDEX "index_indicators_on_objective_id" ON "indicators" ("objective_id");
CREATE INDEX "index_indicators_on_responsible_id" ON "indicators" ("responsible_id");

为什么在objective_id和responsible_id上??没有外键约束？
难道我做错了什么？