models.rb

class Slide < ApplicationRecord
  has_one :slide_content, dependent: :destroy
  has_one :image, through: :slide_content, source: :content, source_type: Image
end

class SlideContent < ApplicationRecord
  belongs_to :slide
  belongs_to :content, polymorphic: true
end

class Image < ApplicationRecord
  has_one :slide_content, as: :content
end

schema.rb

ActiveRecord::Schema.define(version: 20180123152241) do

  create_table "images", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "slide_contents", force: :cascade do |t|
    t.integer  "slide_id"
    t.integer  "content_id"
    t.string   "content_type"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
  end

  add_index "slide_contents", ["slide_id", "content_id", "content_type"], name: "slide_contents_index"

  create_table "slides", force: :cascade do |t|
    t.string   "title"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

end