Listing 57. Adding a picture URL.
URLを入力する領域を追加。
Listing 58. A partial for picture entries.
URLを表示させるpartialを追加。
Listing 59. Adding an image URL to the Post model
attr_accessorにimage_urlを追加。
Listing 60. Conditional logic in the view.
pictureありとなしとでpartialを分ける処理を実装（仮実装）
Listing 61. The view code we'd like to write.
exhibitを使ってpartialをif文で実装することをやめた
Listing 62. Specifying an Exhibit for a picture post.
pictureの投稿におけるテストを追加。
Listing 63. Implementing the picture post Exhibit.
Specを満たすexhibitを実装。
Specifying an Exhibit for a text post.
picture postのspecを真似てtext postのspecを書く。
Listing 64. An Exhibit for text posts.
text postにおけるexhibitを実装