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Fight against Polymorphic Associations

Ruby on Rails


1. Environment

  • rails 5.2.3
  • ruby 2.6.2
  • MacOS Version 10.14.6
  • virtus is for search form

2. Requirements

Add username to the search form item in a auto-sent mail history page.

3. Tables

* Partially picked

3-1. mail_events

id email
1 user@example.com
2 office@example.com

3-2. mail_event_sent_to

id mail_event_id type holder_id office_id
1 1 User 1 1
2 2 Office 1 1

3-3. offices

id name email
1 Office01 office01@example.com

3-4. users

id office_id name
1 1 User01

4. Relations

* Class name is non-existent.

4-1. MailEvent

class MailEvent < ApplicationRecord
  has_many :mail_event_sent_to, foreign_key: :mail_event_id, inverse_of: :mail_event, dependent: :destroy

4-2. MailEventSentTo

class MailEventSentTo < ApplicationRecord
  belongs_to :mail_event
  belongs_to :sent_to, polymorphic: true
  belongs_to :office

4-3. Office

class Office < ApplicationRecord
  has_many :users

4-4. User

class User < ApplicationRecord
  has_many :entrance_and_exits

5. Troubles

  1. Normally, I should define a scope in a class MailEventSentTo class relates to. However, sent_to attribute is related in polymorphic: true, so it does not exist as SentTo class.
  2. type and holder_id defines a receiver(e.g. User + 1 is a user in users table where id is equal to 1. Office + 1 is a user in offices table where id is equal to 1). So the keyword input in the search form must run SQL which searches both users table andoffices table.

6. Implementation

* Class name is non-existent.

6-1. Views

Add sent_to to form_with

<%= search_form_field(f, :sent_to, 'name') %>

6-2. Controllers

Add sent_to to the white list of Strong Parameter.

class MailEventsController < ApplicationController
  def index
    @offices = Office.all
    @search_form = MailEvent::SearchForm.new(search_params)
    @mail_events = @search_form.search...


  def search_params
    params.fetch(:mail_event_search_form, {}).permit(..., :sent_to)

Add sent_to as String data in the class for search.
search method calls a scope MailEvent#sent_to.

class MailEvent::SearchForm < SearchForm::Base
  attribute :sent_to, String

  def search

6-3. Models

Call MailEventSentTo#sent_to via MailEvent#sent_to when search is done with a name. * I would originally like to make an inner join with joins(:mail_event_sent_to) in MailEvent::SearchForm#search methods and directly implement a search logic. But some MailEvent does not have sent_to. That is why the logic looks complicated and strange a little.

class MailEvent < ApplicationRecord
  scope :sent_to, ->(name) {
    return if name.blank?

Call MailEventSentTo#sent_to.

class MailEventSentTo < ApplicationRecord
  scope :sent_to, ->(name) {
    return if name.blank?
    office_ids = Office.like_office_name(name).pluck(:id)
    user_ids = User.like_user_name(name).pluck(:id)
    search_office_ids = where(type: 'Office').where(holder_id: office_ids)
    search_user_ids = where(type: 'User').where(holder_id: user_ids)
    sql = search_office_ids.or(search_user_ids).select(:id).to_sql
    where("#{table_name}.id IN (#{sql})")

What MailEventSentTo#sent_to does are:

Select that records from offices table which include the keyword input in the search form and assign projection of id column to office_ids variable.

office_ids = Office.like_office_name(name).pluck(:id)

Select that records from users table which include the keyword input in the search form and assign projection of id column to user_ids variable.

user_ids = User.like_user_name(name).pluck(:id)

Select all records where type is Office in mail_event_sent_to table and where holder_id is office_ids.
Then assign them to search_office_ids variable.

search_office_ids = where(type: 'Office').where(holder_id: office_ids)

Select all records where type is User in mail_event_sent_to table and where holder_id is user_ids.
Then assign them to search_user_ids variable.

search_user_ids = where(mail_address_holder_type: 'User').where(mail_address_holder_id: user_ids)

Take logical disjunction of search_office_ids and search_user_ids, extract foreign_key, then convert it to SQL. Then assign them to sql variable.

sql = search_office_ids.or(search_user_ids).select(:id).to_sql

Select records which are equal to the sql from mail_events.

where("#{table_name}.id IN (#{sql})")

Office#like_office_name and User#like_user_name are:

  1. Call arel_table with a receiver, Office or User, and load name column as an instance of Arel::Table.
  2. Sanitize the keyword input in the search form and convert it into SQL, which is asssigned to LIKE phrase.
  3. Issue SQL which selects all records where the keyword is included.

7. Conclusion

Do you remember the requirements of this task?
It was just to add name section to a search form in an index page.
I thought that I could make it done soon because I viewed it as a simple task, but it was actually not.

The complicated table and SendGrid makes this task a lot profound one.
I asked review so many times after a lot of tries and errors, and finally made it.
I thought of writing raw SQL, but the code managed to ride on Rails Way.

This task made me understand: