huhn511/_model_form.html.erb

## _model_form.html.erb
<div class="field">
  <%= form.label :image %>
  <%= form.file_field :image %>
</div>

## model_controller.rb
def model_params
  params.require(:model).permit(:image, ...)
end

## model_show.html.erb
<%= image_tag @model.image %>

## model_with_image.rb
class Model < ApplicationRecord
    has_one_attached :image
end
	<div class="field">
	<%= form.label :image %>
	<%= form.file_field :image %>
	</div>
	def model_params
	params.require(:model).permit(:image, ...)
	end