Arel Extra

This extra adds a specialized pagination for collections from sql databases with GROUP BY clauses, by computing the total number of results with COUNT(*) OVER (). It was tested against MySQL (8.0.17) and Postgres (11.5). Before using in a different database, make sure the sql COUNT(*) OVER () performs a count of all the lines after the GROUP BY clause is applied.

Synopsis

See extras for general usage info.

In the pagy.rb initializer:

require 'pagy/extras/arel'

In a controller:

@pagy_a, @items   = pagy_arel(a_collection, ...)

# independently paginate some other collections as usual
@pagy_b, @records = pagy(some_scope, ...)

Files

• arel.rb

Methods

This extra adds the pagy_arel method to the Pagy::Backend to be used in place (or in parallel) of the pagy method when you want to paginate a collection with sql GROUP BY clause, where the number of groups/rows is big (for instance when group by day of the last 3 years). It also adds a pagy_arel_get_variables sub-method, used for easy customization of variables by overriding.

Notice: there is no pagy_arel_get_items method to override, since the items are fetched directly by the specialized pagy_arel method.

pagy_arel(collection, vars=nil)

This method is the same as the generic pagy method, but with improved speed for SQL GROUP BY collections. (see the pagy doc)

pagy_arel_get_vars(collection)

This sub-method is the same as the pagy_get_vars sub-method, but it is called only by the pagy_arel method. (see the pagy_get_vars doc).

pagy_arel_count(collection)

This sub-method it is called only by the pagy_get_vars method. It will detect which query to perform based on the active record groups (sql GROUP BYs). In case there aren’t group values performs a normal .count(:all), otherwise it will perform a COUNT(*) OVER (). The last tells database to perform a count of all the lines after the GROUP BY clause is applied.