The key to pagination

Arrays returned from paginating finds are, in fact, instances of this little class. You may think of WillPaginate::Collection as an ordinary array with some extra properties. Those properties are used by view helpers to generate correct page links.

WillPaginate::Collection also assists in rolling out your own pagination solutions: see create.

If you are writing a library that provides a collection which you would like to conform to this API, you don’t have to copy these methods over; simply make your plugin/gem dependant on the “mislav-will_paginate“ gem:

gem 'mislav-will_paginate'
require 'will_paginate/collection'

# WillPaginate::Collection is now available for use

Methods

public instance

next_page
offset
out_of_bounds?
previous_page
replace
total_entries=

Attributes

 current_page
 per_page
 total_entries
 total_pages

Public class methods

create (page, per_page, total = nil) {|pager| ...}

Just like new, but yields the object after instantiation and returns it afterwards. This is very useful for manual pagination:

@entries = WillPaginate::Collection.create(1, 10) do |pager|
  result = Post.find(:all, :limit => pager.per_page, :offset => pager.offset)
  # inject the result array into the paginated collection:
  pager.replace(result)

  unless pager.total_entries
    # the pager didn't manage to guess the total count, do it manually
    pager.total_entries = Post.count
  end
end

The possibilities with this are endless. For another example, here is how WillPaginate used to define pagination for Array instances:

Array.class_eval do
  def paginate(page = 1, per_page = 15)
    WillPaginate::Collection.create(page, per_page, size) do |pager|
      pager.replace self[pager.offset, pager.per_page].to_a
    end
  end
end

The Array#paginate API has since then changed, but this still serves as a fine example of WillPaginate::Collection usage.

[show source]

# File lib/will_paginate/collection.rb, line 85
    def self.create(page, per_page, total = nil)
      pager = new(page, per_page, total)
      yield pager
      pager
    end

new (page, per_page, total = nil)

Arguments to the constructor are the current page number, per-page limit and the total number of entries. The last argument is optional because it is best to do lazy counting; in other words, count conditionally after populating the collection using the replace method.

[show source]

# File lib/will_paginate/collection.rb, line 49
    def initialize(page, per_page, total = nil)
      @current_page = page.to_i
      raise InvalidPage.new(page, @current_page) if @current_page < 1
      @per_page = per_page.to_i
      raise ArgumentError, "`per_page` setting cannot be less than 1 (#{@per_page} given)" if @per_page < 1
      
      self.total_entries = total if total
    end

Public instance methods

next_page ()

current_page + 1 or nil if there is no next page

[show source]

# File lib/will_paginate/collection.rb, line 112
    def next_page
      current_page < total_pages ? (current_page + 1) : nil
    end

offset ()

Current offset of the paginated collection. If we’re on the first page, it is always 0. If we’re on the 2nd page and there are 30 entries per page, the offset is 30. This property is useful if you want to render ordinals side by side with records in the view: simply start with offset + 1.

[show source]

# File lib/will_paginate/collection.rb, line 102
    def offset
      (current_page - 1) * per_page
    end

out_of_bounds? ()

Helper method that is true when someone tries to fetch a page with a larger number than the last page. Can be used in combination with flashes and redirecting.

[show source]

# File lib/will_paginate/collection.rb, line 94
    def out_of_bounds?
      current_page > total_pages
    end

previous_page ()

current_page - 1 or nil if there is no previous page

[show source]

# File lib/will_paginate/collection.rb, line 107
    def previous_page
      current_page > 1 ? (current_page - 1) : nil
    end

replace (array)

This is a magic wrapper for the original Array#replace method. It serves for populating the paginated collection after initialization.

Why magic? Because it tries to guess the total number of entries judging by the size of given array. If it is shorter than per_page limit, then we know we’re on the last page. This trick is very useful for avoiding unnecessary hits to the database to do the counting after we fetched the data for the current page.

However, after using replace you should always test the value of total_entries and set it to a proper value if it’s nil. See the example in create.

[show source]

# File lib/will_paginate/collection.rb, line 134
    def replace(array)
      result = super
      
      # The collection is shorter then page limit? Rejoice, because
      # then we know that we are on the last page!
      if total_entries.nil? and length < per_page and (current_page == 1 or length > 0)
        self.total_entries = offset + length
      end

      result
    end

total_entries= (number)

sets the total_entries property and calculates total_pages

[show source]

# File lib/will_paginate/collection.rb, line 117
    def total_entries=(number)
      @total_entries = number.to_i
      @total_pages   = (@total_entries / per_page.to_f).ceil
    end