Problem with eager-loading polymorphic associations on Mongoid 9

[stanley90]

While upgrading Mongoid to 9, I discovered a problem that the relation options got overridden and caused very strange behavior, first reported to Mongoid JIRA.
After investigation, I found out that the problem is in overwriting the relation options here
My temporary patch is to add

@options = @options.dup

to the beginning of that block.

Note: this problem can be reproduced only by invoking the eager-loading before invoking the relation, because there is a memo here

[jclusso]

@stanley90 would you be willing to provide an example project that reproduces this?

[stanley90]

@jclusso Wasn't easy, I originally forgot to mention that there also has to be STI, but here goes (sorry couldn't quickly figure out how to write it as a proper spec)

require 'rubygems'
require 'mongoid'
require 'mongoid_includes'

Mongoid.load_configuration({ clients: { default: { database: 'mi-22', hosts: %w[localhost:27017] } } })

class Main
  include Mongoid::Document
  belongs_to :related, polymorphic: true, optional: true
end

class Related
  include Mongoid::Document
end

class Two < Related; end
class Three < Related; end

case ARGV.first
when 'init'
  Main.destroy_all
  Related.destroy_all

  Main.create!(related: Two.create!)
  Main.create!(related: Three.create!)
when 'good'
  Main.all.entries
when 'bad'
  Main.includes(:related).entries # this overwrites the options in #for_class_name
end

Main.last.related.id

Instructions:

• first run with init arg to only create the records
• then arbitrarily run with good and bad

The details with respective code links are in the initial comment and linked JIRA ticket.

[jclusso]

@stanley90 thanks for this. We'll see if we can't figure out a solution unless you have an idea for one.

[stanley90]

@jclusso my suggestion (and patch in my project) is in the initial comment - dup the options first. There might be other ways, maybe not using instance_eval, don't know.

[danarnold]

@stanley90 I tested your solution and it seems to fix the issue. Did you want to make a PR for it, or should I just do it?

[stanley90]

@danarnold You can do it if you have the time, also I had a hard time writing a spec for it.