# Pagy

In 
Core

The scope of the Pagy class is keeping track of the all integers and variables involved in the pagination. It basically takes a few integers (such as the count of the collection, the page number, the items per page, ...), does some simple arithmetic and creates a very small object that allocates less than 3k of memory (source).

# Synopsis

pagy.rb (initializer)
# set global defaults and extra variables
# they will get merged with every new Pagy instance
Pagy::DEFAULT[:items]    = 25
Pagy::DEFAULT[:some_var] = 'default value'

# create a new instance (usually done automatically by the #pagy controller method)
pagy = Pagy.new(count: 1000, page: 3, instance_var: 'instance var')
#=> #<Pagy:0x000056070d954330 ... >

# fetch variables (with global default)
pagy.vars[:some_var]
#=> "default value"

# fetch variables passed to the instance
pagy.vars[:instance_var]
#=> "instance var"

# fetch instance attributes
pagy.items
#=> 25

# get the page series
pagy.series
#=> [1, 2, "3", 4, 5, 6, 7, :gap, 40]

# Global Default

The Pagy::DEFAULT is a globally available hash used to set defaults variables. It is a constant for easy access, but it is mutable to allow customizable defaults. You can override it in the pagy.rb initializer file in order to set your own application global defaults. After you are done, you should freeze it, so it will not change accidentally. It gets merged with the variables passed with the new method every times a Pagy instance gets created.

You will typically use it in a pagy.rb initializer file to pass defaults values. For example:

pagy.rb (initializer)
Pagy::DEFAULT[:items]     = 25
Pagy::DEFAULT[:my_option] = 'my option'
...
Pagy::DEFAULT.freeze

# Methods

This method returns the pathname of the pagy/lib root dir. It is useful to get the absolute path of locale and javascript files installed with the gem.

Notice: If you use the Pagy::Backend its pagy method will instantiate and return the Pagy object for you.

The Pagy.new method accepts a single hash of variables that will be merged with the Pagy::DEFAULT hash and will be used to create the object.

This method is the core of the pagination. It returns an array containing the page numbers and the :gap items to be rendered with the navigation links (e.g. [1, :gap, 7, 8, "9", 10, 11, :gap, 36]). It accepts an optional size keyword argument (only useful for extras), defaulted on the :size variable.

The nav helpers basically loop through this array and render the correct item by simply checking its type:

  • if the item is an Integer then it is a page link
  • if the item is a String then it is the current page
  • if the item is the :gap symbol then it is a gap in the series

That is self-contained, simple and efficient.

Label for the current page. Its only function in the Pagy class is supporting the API of various frontend methods that require labelling for Pagy::Calendar instances. It returns the current page label that will get displayed in the helpers.

Label for any page. Its only function in the Pagy class is supporting the API of various frontend methods that require labelling for Pagy::Calendar instances. It returns the page label that will get displayed in the helpers.

# Variables

All the variables passed to the new method will be merged with the Pagy::DEFAULT hash and will be kept in the object, passed around with it and accessible through the pagy.vars hash.

They can be set globally by using the Pagy::DEFAULT hash or passed to the Pagy.new method and are accessible through the vars reader.

Notice: Pagy replaces the blank values of the passed variables with their default values coming from the Pagy::DEFAULT hash. It also applies to_i on the values expected to be integers, so you can use values from request params without problems. For example: pagy(some_scope, items: params[:items]) will work without any additional cleanup.

# Instance Variables

A few variables are particularly important for the calculation of the pagination, and therefore are validated and used to initialize a few instance variables.

The only mandatory instance variable to be passed is the :count of the collection to paginate: all the other variables are optional and have sensible defaults. Of course you will also have to pass the page or you will always get the default page number 1. They are all integers:

Variable Description Default
:count The total count of the collection to paginate (mandatory argument) nil
:page The requested page number 1
:items The requested number of items for the page 20
:outset The initial offset of the collection to paginate: pass it only if the collection had an offset 0

# Other Variables

Variable Description Default
:size The size of the page links to show: can be an array of initial pages, before current page, after current page, final pages or the total page size. (see also How to control the page links) 7
:page_param The name of the page param name used in the url. (see How to customize the page param) :page
:params It can be a Hash of params to add to the URL, or a Proc that can edit/add/delete the request params (see How to customize the params) {}
:fragment The arbitrary fragment string (including the "#") to add to the url. (see How to customize the params) ''
:link_extra The extra attributes string (formatted as a valid HTML attribute/value pairs) added to the page links (see How to customize the link attributes) ''
:item_i18n_key The i18n key to lookup the item_name that gets interpolated in a few helper outputs (see How to customize the item name) 'pagy.item_name'
:nav_i18n_key The i18n key to lookup the aria-label for the pagy*_nav helpers 'pagy.aria_label.nav'
:cycle Enable cycling/circular/infinite pagination: true sets next to 1 when the current page is the last page false
:request_path Allows overriding the request path for pagination links. Pass the path only (not the absolute url). (see Customize the request path) request.path
jsonapi Enable jsonapi compliance of the pagy query params false
count_args The arguments passed to the collection.count. You may want to set it to [] in ORMs different than ActiveRecord [:all]

There is no specific validation for non-instance variables.

# Attribute Readers

Pagy exposes all the instance variables needed for the pagination through a few attribute readers. They all return integers ( or nil), except the vars hash:

Reader Description
count The collection :count
page The current page number
items The requested number of items for the page
pages The number of total pages in the collection (same as last but with cardinal meaning)
in The number of the items in the page
last The number of the last page in the collection (same as pages but with ordinal meaning)
offset The number of items skipped from the collection in order to get the start of the current page (:outset included)
from The collection-position of the first item in the page (:outset excluded)
to The collection-position of the last item in the page (:outset excluded)
prev The previous page number or nil if there is no previous page
next The next page number or nil if there is no next page
vars The variables hash
params The :params variable (Hash or Proc)
request_path The request path used for pagination helpers. If blank, helpers will use request.path

# Lowest limit analysis

The lowest possible limit of the pagination is reached when the collection has 0 count. In that case the Pagy object created has the following peculiar attributes:

Attribute Value
count 0
page 1
pages 1
last 1
in 0
from 0
to 0
prev nil
next nil
series ["1"]

Which means:

  • there is always a page #1 in the pagination, even if it's empty
  • pages and last are always at least both 1
  • the series array contains always at least the page #1, which for a single page is also the current page, thus a string. With size: [] the series method returns []
  • in, from and to of an empty page are all 0
  • prev and next of a single page (not necessary an empty one) are both nil (i.e. there is no other page)

# Exceptions

A subclass of ArgumentError that offers a few information for easy exception handling when some of the variable passed to the constructor is not valid.

rescue Pagy::VariableError => e
e.pagy #=> the pagy object that raised the exception
e.variable #=> the offending variable symbol (e.g. :page)
e.value #=> the value of the offending variable (e.g -3)
end

Mostly useful if you want to rescue from bad user input (e.g. illegal URL manipulation).

A subclass of Pagy::VariableError: it is raised when the :page variable exceeds the maximum possible value calculated for the :count, i.e. the :page variable is in the correct range, but it's not consistent with the current :count. That may happen when the :count has been reduced after a page link has been generated (e.g. some record has been just removed from the DB). See also the overflow extra.