Error: Validation failed: Images imageable must exist , rails-5.0 , paperclip-5 - paperclip

Question

While i was trying to submit the form, following error occured: Validation failed: Images imageable must exist and render the same new.html.erb view.
If i comment the file field in new.html.erb. Product is being created successfully.
ProductsController:
def new
#product = Product.new
end
def create
#product = Product.create!(product_params)
if #product.save
redirect_to products_path, notice: "Product Created Successfully"
else
render "new"
end
end
def product_params
params.require(:product).permit(:name, :quantity, :price, images_attributes: [:id, :photo, :_destroy])
end
new.html.erb:
<%= nested_form_for #product, html: { multipart: true } do |f|%>
<h2>New</h2>
<P> <%= f.label :name %> <%= f.text_field :name %> </P>
<P> <%= f.label :quantity %> <%= f.text_field :quantity %> </P>
<P> <%= f.label :price %> <%= f.text_field :price %> </P>
<%= f.fields_for :images do |p| %>
<p> <%= p.label :photo %> <%= p.file_field :photo %> </p>
<%= p.link_to_remove "Remove Image" %>
<% end %>
<%= f.link_to_add "Add Image", :images %>
<%= f.submit "Add Product" %>
<% end %>
20160725102038_add_image_columns_to_imageable.rb:
class AddImageColumnsToImageable < ActiveRecord::Migration[5.0]
def up
add_attachment :images, :photo
end
def down
remove_attachment :images, :photo
end
end
Model:product.rb
class Product < ApplicationRecord
has_one :variant
has_many :images, as: :imageable, dependent: :destroy
accepts_nested_attributes_for :images, allow_destroy: true
end
Model:image.rb
class Image < ApplicationRecord
belongs_to :imageable, polymorphic: true
has_attached_file :photo, styles: { medium: "300x300>", thumb: "100x100>" }, default_url: "/images/:style/missing.png"
validates_attachment :photo, content_type: { content_type: ["image/jpg", "image/jpeg", "image/png", "image/gif"] }
end

Answer 1

In rails 5, belongs_to makes sure that the associated model must exist.
E.g In this polymorphic association, Image model has belongs_to :imageable and Product model has has_many :images.
So here in new.html.erb we are creating an image, but respective product not exist, so that's why error Image imageable must exist .
Solution
Add optional: true while making an association of belong_to in Image model.
Image Model now looks like:
class Image < ApplicationRecord
belongs_to :imageable, polymorphic: true, optional: true
has_attached_file :photo, styles: { medium: "300x300>", thumb: "100x100>" }, default_url: "/images/:style/missing.png"
validates_attachment :photo, content_type: { content_type: ["image/jpg", "image/jpeg", "image/png", "image/gif"] }
end