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

Question

I\'m new in Rails... smile

In my blog aplication I want to have a \"Previous post\" link and a \"Next post\" link in the bottom of my show view.

How do I do

Answer 1

You really just need to run 2 queries, one for each of "prev" and "next". Lets assume you have a created_at column.

Psuedo-code:

# get prev
select * from posts where created_at < #{this_post.created_at} order by created_at desc limit 1

# get next
select * from posts where created_at > #{this_post.created_at} order by created_at desc limit 1

Of course "this_post" is the current post.

If your posts are stored with an auto_increment column and you dont re-use IDs you can just use the id column in place of created_at - the id column should already be indexed. If you want to use the created_at column then you will definitely want to have an index on that column.

Answer 2

If each title is unique and you need alphabetical, try this in your Post model.

def previous_post
  self.class.first(:conditions => ["title < ?", title], :order => "title desc")
end

def next_post
  self.class.first(:conditions => ["title > ?", title], :order => "title asc")
end

You can then link to those in the view.

<%= link_to("Previous Post", @post.previous_post) if @post.previous_post %>
<%= link_to("Next Post", @post.next_post) if @post.next_post %>

Untested, but it should get you close. You can change title to any unique attribute (created_at, id, etc.) if you need a different sort order.

Answer 3

Give the will_paginate Gem a try. It provides all the features you need to paginate your post entries. learn here too

You can look at here too for example code if you want add next and previous buttons.

Answer 4

This is how I did it. Firstly, add a couple of named scopes to your Post model:

def previous
  Post.find_by_id(id - 1, :select => 'title, slug etc...')
end

def next
  Post.find_by_id(id + 1, :select => 'title, slug etc...')
end

Note the use of the :select option to limit the fields because you probably don't want to retrieve a fully-populated Post instance just for showing the links.

Then in my posts_helper I have this method:

def sidebar_navigation_links
  next_post = @post.next
  previous_post = @post.previous
  links = ''
  if previous_post
    links << content_tag(:h3, 'Previous')
    links << content_tag(:ul, content_tag(:li,
                              content_tag(:a, previous_post.title,
                                          :href => previous_post.permalink)))
  end
  if next_post
    links << content_tag(:h3, 'Next', :class => 'next') if previous_post
    links << content_tag(:h3, 'Next') if previous_post.nil?
    links << content_tag(:ul, content_tag(:li,
                              content_tag(:a, next_post.title,
                                          :href => next_post.permalink)))
  end
  content_tag(:div, links)
end

I'm sure this could be refactored to be less verbose, but the intent is clear. Obviously your markup requirements will be different to mine, so you may not choose to use an unordered list, for example.

The important thing is the use of the if statements because if you're on the first post then they'll be no previous post and conversely, if you're on the last post they'll be no next post.

Finally, simply call the helper method from your view:

<%= sidebar_navigation_links %>

Answer 5

I've created gem proximal_records especially for this kind of task and it works on any dynamically created scope in your model.

https://github.com/dmitry/proximal_records

Basic example:

class Article < ActiveRecord::Base
  include ProximalRecords
end


scope = Article.title_like('proximal').order('created_at DESC, title ASC')
current_record = scope.to_a[5]
p, n = current_record.proximal_records(scope) # will find record 5 and 7

Answer 6

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.