Rails在表单中的多态关联

Question

This is a slightly unique version of a polymorphic association. It's one of those "real world" problems that I'm struggling to solve and haven't come across many good answers so I made my own.

A Transaction record has many Tasks and each Task has an Assignee , which can be from multiple tables.

# Models
class Transaction < ApplicationRecord
  has_many :tasks
  has_many :borrowers
  has_many :partners

  # Combine into a single array to display on the collection_select
  def assignees
    borrowers + partners
  end
end

class Task < ApplicationRecord
  # has attribute :assignee_type_and_id (string)

  belongs_to :transaction

  # Reverse engineer single attribute into type/id parts
  def assignee
    if assignee_type_and_id
      parts = assignee_type_and_id.split(".")
      type = parts.first
      id = parts.last

      if type.to_s.downcase == "borrower"
        Borrower.find(id)
      elsif type.to_s.downcase == "partner"
        Partner.find(id)
      end
    end
  end
end

class Borrower < ApplicationRecord
  # has attribute :name
  belongs_to :transaction

  def type_and_id
    "borrower.#{id}"
  end
end

class Partner < ApplicationRecord
  # has attribute :name
  belongs_to :transaction

  def type_and_id
    "partner.#{id}"
  end
end

On the Task form pages, I want a single HTML select that combines BOTH the Borrowers and Partners .

Classic polymorphism says to add an assignee_type column, but now I'm working with 2 fields instead of one.

My solution is to combine these 2 into a single select such that the final value is of the format assignee_type.assignee_id .

# form.html.erb
<%= form_for @task do |f| %>
  <%= f.label :assignee_type_and_id, "Assignee" %>

  <%= f.collection_select :assignee_type_and_id, @transaction.assignees, :name, :type_and_id %>
<% end %>

When the form is submitted, it POSTs values in the format borrower.123 , partner.57 , etc, and that value gets stored in the DB column.

When I want to retrieve the actual task's Assignee , I have to do a little reverse engineering as noted above in the Task#assignee method.

Question

Is there a more appropriate way to do this? I came up with this myself, which scares me because I know problems like this must have been solved by people much smarter than me...

Is there a way to make this work with "normal" polymorphism instead of forcing my own hybrid version?

Update

I happened upon Rails 4.2+ GlobalID, which seems to do this very thing. Unless there's a reason not to use that, I may use that implementation instead of my own "bastardized" version. Is there any better solution to a situation like this?

Answer 1

For these type of problems where a form spans multiple models/complex associations I use a form backing object. It keeps everything clean and modular. Here is a good write up: https://content.pivotal.io/blog/form-backing-objects-for-fun-and-profit