Rails和多态关联

Question

I have three models: User , Company , and Subscription . What I am trying to accomplish is a Subscription belongs to either a User OR a Company .

To try accomplish this, I referenced this guide , but I have been unsuccessful as the record creation keeps rolling back.

here's my Company model:

# app/models/company.rb
class Company < ApplicationRecord
    has_many :subscriptions, dependent: :destroy, as: :imageable
end

here's my User model:

# app/models/user.rb
class User < ApplicationRecord
  has_many :subscriptions, dependent: :destroy, as: :imageable
end

and finally, here's my Subscription model:

class Subscription < ApplicationRecord
    belongs_to :imageable, polymorphic: true
end

Now as far as the migration file, this is my Subscription migration file:

class CreateSubscriptions < ActiveRecord::Migration[5.1]
  def change
    create_table :subscriptions do |t|
      t.references :imageable, polymorphic: true, index: true
      t.date :start_date
      t.date :stop_date

      t.timestamps
    end
  end
end

As far as what I can see, this is pretty much exactly like the guide shows, but it keeps rolling back. Here's the output of the rails console:

Loading development environment (Rails 5.1.6)
2.5.1 :001 > Subscription.create(imageable_id: 1, start_date: Time.now, stop_date: 2.days.from_now)
   (8.6ms)  SET NAMES utf8,  @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'),  @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483
   (0.2ms)  BEGIN
   (0.3ms)  ROLLBACK
 => #<Subscription id: nil, imageable_type: nil, imageable_id: 1, start_date: "2018-10-10", stop_date: "2018-10-12", created_at: nil, updated_at: nil> 
2.5.1 :002 > 

Here are the questions that I have:

1. Why is there an imageable_type field? Is that created by t.references and if so, do I need this? Can I just use imageable_id instead of t.references as the other part of the suggestion shows?
2. Why is it rolling back? Are polymorphic associations done differently in Rails 5.x or something by chance?
3. According to the graph shown in the guide, it looks like if a picture belongs to imageable_id 4, then if there is an employee AND a production with the ID of 4, then a picture would belongs to both instead of one or the other like I'm trying to accomplish. Correct?

Answer 1

For polymorphic association, you should also pass imageable_type along with imageable_id . You don't do it and that's why it doesn't work, most probably (ie there might be other reasons, I don't know, but this one is pretty obvious).

imageable_type holds the name of the class of the record given Subscription is associated to.

Answer 2

In your association, Imageable type will contain the class name and imageble id will contain the id of that class. So if you want to create subscription for user you can do like below

User.first.subcriptions.create(start_date: Time.now, stop_date: 2.days.from_now)

So it will automatically pick up First user's id in imageable id and take "User" as imageable type.

If you want to create subscription manually, you must have to pass both fields imageable type and imageble id like below,

Subscription.create(imageable_id: 1, imageable_type: "User", start_date: Time.now, stop_date: 2.days.from_now)

1. Why is there an imageable_type field? Is that created by t.references and if so, do I need this? Can I just use imageable_id instead of t.references as the other part of the suggestion shows?

=> imageable_type will contain the class of associate model like "User" or "Company"

2. Why is it rolling back? Are polymorphic associations done differently in Rails 5.x or something by chance?

=> No, you setup it correctly

3. According to the graph shown in the guide, it looks like if a picture belongs to imageable_id 4, then if there is an employee AND a production with the ID of 4, then a picture would belongs to both instead of one or the other like I'm trying to accomplish. Correct ?

=> It depends on both imageable_id and imageble_type , so by combination of both this you will get record. If imageable_id is 4 and imageable_type is "Picture" then it will take Picture with id 4.

Please check this link for understaing