Common in index method

Get all posts and save them to @post, ordered by when they were creating in descending order:

@post = Post.all.order("created_at DESC")

Get all posts by a particular user and save them to @post, ordered by when they were creating in descending order:
@post = current_user.posts("created_at DESC")

Common in new method

Create a new post for the current user when that user has_many posts:

@post = current_user.posts.build

Create a new post for the current user when that user only has_one post:

@post = current_user.build_post

To authorize which users can access what

See my post here.