in elasticsearch-model/lib/elasticsearch/model/response/pagination/will_paginate.rb [53:62]
def paginate(options)
param_name = options[:param_name] || :page
page = [options[param_name].to_i, 1].max
per_page = (options[:per_page] || __default_per_page).to_i
search.definition.update size: per_page,
from: (page - 1) * per_page
self
end