rails: ActiveStorage contaminating model instances resulting in ActiveRecord::AssociationTypeMismatch

Using ActiveStorage can result in the class instance becoming contaminated in a strange way that results in a ActiveRecord::AssociationTypeMismatch exception when trying to associate the instance with another ActiveRecord model.

This example may seem a little unconventional as I discovered this issue whilst building seeds for my application.

Steps to reproduce

require 'open-uri'

class User
  has_one_attached :avatar
  has_many :talks
end

class Talk
  belongs_to :user
end

user_a = User.create!({ email: 'user.a@example.com' })
user_b = User.create!({ email: 'user.b@example.com' })

user_a.avatar.attach(io: open('https://example.com/any/valid/image/uri'))

user_a.class.to_s
# => "User"

user_b.class.to_s
# => "User"

Talk.new({ user: user_a })
# => ActiveRecord::AssociationTypeMismatch

Talk.new({ user: user_b })
# => true

Expected behavior

Associating user_a to a new instance of Talk should return true.

Actual behavior

The following exception is unexpectedly raised:

ActiveRecord::AssociationTypeMismatch (User(#70172285884400) expected, got #<User id: "5c01db01-fd4d-464c-bb36-33d29469fe77", email: "example@example.com", created_at: "2019-10-29 21:20:16", updated_at: "2019-10-29 21:20:45"> which is an instance of User(#70172283718280))

Further information

I added a binding.pry into the method raise_on_type_mismatch! in activerecord-6.0.0/lib/active_record/associations/association.rb. I’ve included the output below:

From: /Users/adambutler/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activerecord-6.0.0/lib/active_record/associations/association.rb @ line 275 ActiveRecord::Associations::Association#raise_on_type_mismatch!:

    273: def raise_on_type_mismatch!(record)
    274:   unless record.is_a?(reflection.klass)
 => 275:     binding.pry
    276:     fresh_class = reflection.class_name.safe_constantize
    277:     unless fresh_class && record.is_a?(fresh_class)
    278:       message = "#{reflection.class_name}(##{reflection.klass.object_id}) expected, "\
    279:         "got #{record.inspect} which is an instance of #{record.class}(##{record.class.object_id})"
    280:       raise ActiveRecord::AssociationTypeMismatch, message
    281:     end
    282:   end
    283: end

[1] pry(#<ActiveRecord::Associations::BelongsToAssociation>)> record.is_a?(reflection.klass)
=> false

[2] pry(#<ActiveRecord::Associations::BelongsToAssociation>)> record
=> #<User id: "a19c59f1-fd39-40da-abd8-42bc94f2bfc7", email: "user@example.com", created_at: "2019-10-29 21:32:33", updated_at: "2019-10-29 21:32:34">

[3] pry(#<ActiveRecord::Associations::BelongsToAssociation>)> record.class
=> User(id: uuid, email: string)

[4] pry(#<ActiveRecord::Associations::BelongsToAssociation>)> reflection.klass
=> User(id: uuid, email: string)

System configuration

Rails version: 6.0.0

Ruby version: 2.6.5