Exercise.rb

class Exercise < ApplicationRecord
	has_one :image, :as =>:imageable
	accepts_nested_attributes_for :image
end

Exercise_spec.rb

# This is failing
describe 'saving image' do
	context 'with an associated image' do

		let :exercise do
			build :exercise
		end
		
		before :each do
			exercise.build_image(attributes_for(:image))
		end

		it 'should save correctly with an associated image' do
			expect{ exercise.save! }.to_not raise_error
		end
  end
end

ExercisesController.rb

class Admin::ExercisesController < Admin::BaseController
  def new
    @exercise = Exercise.new
    @exercise.build_image
  end

  def create
    @exercise = Exercise.new(exercise_params)
    respond_to do |format|
      if @exercise.save
        format.html { redirect_to admin_exercises_url, notice: 'Exercise was successfully created.' }
      else
        format.html { render :new }
      end
    end
  end

  private
    def exercise_params
      params.require(:exercise).permit(:name, :exercise_category_id, :image_attributes => [:file])
    end
end

Image.rb

class Image < ApplicationRecord
	belongs_to :imageable, polymorphic: true
end

pry.rb

[25] pry(main)> exercise = Exercise.new(:name =>"anything")
=> #<Exercise:0x0055af50e06870
 id: nil,
 name: "anything",
 created_at: nil,
 updated_at: nil,
 exercise_category_id: nil>
[26] pry(main)> exercise.build_image(:file_file_name => "anything")
=> #<Image:0x0055af50d806f8
 id: nil,
 title: "",
 imageable_type: "Exercise",
 imageable_id: nil,
 position: 1,
 created_at: nil,
 updated_at: nil,
 file_file_name: "anything",
 file_content_type: nil,
 file_file_size: nil,
 file_updated_at: nil>
[27] pry(main)> exercise.save!
ActiveRecord::RecordInvalid: Validation failed: Image imageable must exist
from /home/joshua/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0/lib/active_record/validations.rb:78:in `raise_validation_error'