gistfile1.rb

class User
  has_many :entries
end

class Entry
  has_many :images
  belongs_to :user
end

class Image
  has_attached_image :image
  belongs_to :entry
end

##########################


def upload
  @entry = Entry.find(…)
  @image = @entry.images.new(params[:…])
  @image.save
  …
end