Weird behavior...

base_job.rb

class BaseJob < ActiveRecord::Base

  belongs_to :child_job, polymorphic: true

  has_attached_file :header_image, :styles => { :medium => "800x500#" }
  validates_attachment_content_type :header_image, :content_type => /\Aimage\/.*\Z/

  validates :title, presence: true
  validates :prompt_html, presence: true

end

link_job.rb

class LinkJob < ActiveRecord::Base
  include BaseJobMethods

  has_one :base_job, as: :child_job
  has_many :job_completions, as: :job
  belongs_to :redirector

  accepts_nested_attributes_for :base_job

  validates :base_job, presence: true
  validates :redirector, presence: true

  
  def completed_by_user?(user)
    false if user.nil?
    user.has_completed_job?(self)
  end

  def has_user_met_requirements?(user)
    false if user.nil?
    user.redirector_clicks.where(redirector_id: redirector.id).exists?
  end

end

link_job_spec.rb

require 'rails_helper'

RSpec.describe LinkJob, type: :model do
  it { should validate_presence_of :base_job }
  it { should validate_presence_of :redirector }
  it "should return true when asked if it is #persisted?" do
    link_job = create(:link_job)
    expect(LinkJob.count).to eq(1)
    expect(link_job.redirector.persisted?).to eq(true)
    expect(link_job.base_job.persisted?).to eq(true)
    expect(link_job.new_record?).to eq(false)
    expect(link_job.destroyed?).to eq(false)
    expect(link_job.persisted?).to eq(true) # fails here...
  end
end

redirector.rb

class Redirector < ActiveRecord::Base
  has_many :redirector_clicks
  has_one :link_job
end

(The thing that led me to this was the fact that the form submission caused what looked like a duplicate record, but in fact was due to the form having the new link_jobs path set on it, then I tracked it down to the persisted method by looking at the form_for helper itself to determine which path it drops into resource forms.)

As it turns out, the include BaseJobMethods line referenced a module that included ActiveRecord::Base. Removing that inclusion fixed the issue.