Rails: "Next post" and "Previous post" links in my show view, how to?

My method will allow you to automatically use model scopes. For example, you may only want to display posts that are "published."

In your model:

def self.next(post)
  where('id < ?', post.id).last
end

def self.previous(post)
  where('id > ?', post.id).first
end

In your view

<%= link_to 'Previous', @posts.previous(@post) %>
<%= link_to 'Next', @posts.next(@post) %>

In your controller

@photos = Photo.published.order('created_at')

Associated RSpec tests:

describe '.next' do
  it 'returns the next post in collection' do
    fourth_post = create(:post)
    third_post = create(:post)
    second_post = create(:post)
    first_post = create(:post)

    expect(Post.next(second_post)).to eq third_post
  end

  it 'returns the next post in a scoped collection' do
    third_post = create(:post)
    decoy_post = create(:post, :published)
    second_post = create(:post)
    first_post = create(:post)

    expect(Post.unpublished.next(second_post)).to eq third_post
  end
end

describe '.previous' do
  it 'returns the previous post in collection' do
    fourth_post = create(:post)
    third_post = create(:post)
    second_post = create(:post)
    first_post = create(:post)

    expect(Post.previous(third_post)).to eq second_post
  end

  it 'returns the previous post in a scoped collection' do
    third_post = create(:post)
    second_post = create(:post)
    decoy_post = create(:post, :published)
    first_post = create(:post)

    expect(Post.unpublished.previous(second_post)).to eq first_post
  end
end

Note: there will be small issues when you reach the first/last post in a collection. I recommend a view helper to conditionally show the previous or next button only if it exists.