Question

我想在Rails 4中使用STI。我已经有了一个模型Boilerplate，现在我想继承一个BoilerplateOriginal和一个BoilerplateCopy模型。所以我在表格中添加了一列type：

class AddTypeToBoilerplates < ActiveRecord::Migration
  def up
    add_column :boilerplates, :type, :string
    Boilerplate.update_all type: 'BoilerplateOriginal'
    change_column :boilerplates, :type, :string, null: false
  end

  def down
    remove_column :boilerplates, :type
  end
end

可悲的是，这个专栏似乎没有被Rails自动填充：

[1] a4aa2 »  x = Boilerplate.new
=> #<Boilerplate:0x00000101609580> {
               :id => nil,
            :title => nil,
             :type => nil
}
[2] a4aa2 »  x.valid?
  Boilerplate Exists (0.4ms)  SELECT  1 AS one FROM "boilerplates" WHERE "boilerplates"."title" IS NULL LIMIT 1
=> false
[3] a4aa2 »  x.errors
=> {
  :title => [
    [0] "must not be empty"
  ]
}
[4] a4aa2 »  x.title = 'test'
=> "test"
[5] a4aa2 »  x.valid?
  Boilerplate Exists (0.1ms)  SELECT  1 AS one FROM "boilerplates" WHERE "boilerplates"."title" = 'test' LIMIT 1
=> true
[6] a4aa2 »  x.save
   (0.1ms)  begin transaction
  Boilerplate Exists (0.2ms)  SELECT  1 AS one FROM "boilerplates" WHERE "boilerplates"."title" = 'test' LIMIT 1
  SQL (1.4ms)  INSERT INTO "boilerplates" ("title") VALUES (?)  [["title", "test"]]
SQLite3::ConstraintException: NOT NULL constraint failed: boilerplates.type: INSERT INTO "boilerplates" ("title") VALUES (?)
   (0.1ms)  rollback transaction
from /Users/josh/.rvm/gems/ruby-2.1.0@a4aa2/gems/sqlite3-1.3.10/lib/sqlite3/statement.rb:108:in `step'

我是否错过了重要的事情？我认为Rails只是填充了:type列，当它可用时？

Answer 1

好。你做错了当然。只有当您使用父<?xml version="1.0" encoding="UTF-8"?> <works xmlns="http://pbn.nauka.gov.pl/-/ns/bibliography" pbn-unit-id="1388"><article><title>Mukowiscydoza</title></article></works>的任何子类创建对象时，才会设置type列。但是，如果您使用Boilerplate，则需要手动传递Boilerplate.new值。另一方面，当您执行type或BoilerplateCopy.new时，默认情况下，ActiveRecord会将BoilerplateOriginal.new设置为子类的类名。

阅读Single table inheritance的官方文档。

Active Record允许通过将类的名称存储在默认名为type的列中来进行继承（可以通过覆盖type来更改）。这意味着继承看起来像这样：
Base.inheritance_column
当您执行class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end时，此记录将保存在公司表格中Firm.create(name: "37signals")。然后，您可以使用type = “Firm”再次获取此行，它将返回Company.where(name: '37signals').first个对象。

Rails单表继承：类型coumn未自动填充

1 个答案: