1 files changed, 213 insertions, 40 deletions

diff --git a/lib/api/helpers/pagination.rb b/lib/api/helpers/pagination.rb
index 09805049169..3308212216e 100644
--- a/lib/api/helpers/pagination.rb
+++ b/lib/api/helpers/pagination.rb
@@ -2,67 +2,240 @@ module API
   module Helpers
     module Pagination
       def paginate(relation)
-        relation = add_default_order(relation)
+        strategy = if params[:pagination] == 'keyset' && Feature.enabled?('api_keyset_pagination')
+                     KeysetPaginationStrategy
+                   else
+                     DefaultPaginationStrategy
+                   end
 
-        relation.page(params[:page]).per(params[:per_page]).tap do |data|
-          add_pagination_headers(data)
-        end
+        strategy.new(self).paginate(relation)
       end
 
-      private
+      class KeysetPaginationInfo
+        attr_reader :relation, :request_context
 
-      def add_pagination_headers(paginated_data)
-        header 'X-Per-Page',    paginated_data.limit_value.to_s
-        header 'X-Page',        paginated_data.current_page.to_s
-        header 'X-Next-Page',   paginated_data.next_page.to_s
-        header 'X-Prev-Page',   paginated_data.prev_page.to_s
-        header 'Link',          pagination_links(paginated_data)
+        def initialize(relation, request_context)
+          # This is because it's rather complex to support multiple values with possibly different sort directions
+          # (and we don't need this in the API)
+          if relation.order_values.size > 1
+            raise "Pagination only supports ordering by a single column." \
+              "The following columns were given: #{relation.order_values.map { |v| v.expr.name }}"
+          end
 
-        return if data_without_counts?(paginated_data)
+          @relation = relation
+          @request_context = request_context
+        end
 
-        header 'X-Total',       paginated_data.total_count.to_s
-        header 'X-Total-Pages', total_pages(paginated_data).to_s
-      end
+        def fields
+          keys.zip(values).reject { |_, v| v.nil? }.to_h
+        end
 
-      def pagination_links(paginated_data)
-        request_url = request.url.split('?').first
-        request_params = params.clone
-        request_params[:per_page] = paginated_data.limit_value
+        def column_for_order_by(relation)
+          relation.order_values.first&.expr&.name
+        end
 
-        links = []
+        # Sort direction (`:asc` or `:desc`)
+        def sort
+          @sort ||= if order_by_primary_key?
+                      # Default order is by id DESC
+                      :desc
+                    else
+                      # API defaults to DESC order if param `sort` not present
+                      request_context.params[:sort]&.to_sym || :desc
+                    end
+        end
 
-        request_params[:page] = paginated_data.prev_page
-        links << %(<#{request_url}?#{request_params.to_query}>; rel="prev") if request_params[:page]
+        # Do we only sort by primary key?
+        def order_by_primary_key?
+          keys.size == 1 && keys.first == primary_key
+        end
 
-        request_params[:page] = paginated_data.next_page
-        links << %(<#{request_url}?#{request_params.to_query}>; rel="next") if request_params[:page]
+        def primary_key
+          relation.model.primary_key.to_sym
+        end
 
-        request_params[:page] = 1
-        links << %(<#{request_url}?#{request_params.to_query}>; rel="first")
+        def sort_ascending?
+          sort == :asc
+        end
 
-        unless data_without_counts?(paginated_data)
-          request_params[:page] = total_pages(paginated_data)
-          links << %(<#{request_url}?#{request_params.to_query}>; rel="last")
+        # Build hash of request parameters for a given record (relevant to pagination)
+        def params_for(record)
+          return {} unless record
+
+          keys.each_with_object({}) do |key, h|
+            h["ks_prev_#{key}".to_sym] = record.attributes[key.to_s]
+          end
         end
 
-        links.join(', ')
-      end
+        private
+
+        # All values present in request parameters that correspond to #keys.
+        def values
+          @values ||= keys.map do |key|
+            request_context.params["ks_prev_#{key}".to_sym]
+          end
+        end
 
-      def total_pages(paginated_data)
-        # Ensure there is in total at least 1 page
-        [paginated_data.total_pages, 1].max
+        # All keys relevant to pagination.
+        # This always includes the primary key. Optionally, the `order_by` key is prepended.
+        def keys
+          @keys ||= [column_for_order_by(relation), primary_key].compact.uniq
+        end
       end
 
-      def add_default_order(relation)
-        if relation.is_a?(ActiveRecord::Relation) && relation.order_values.empty?
-          relation = relation.order(:id)
+      class KeysetPaginationStrategy
+        attr_reader :request_context
+        delegate :params, :header, :request, to: :request_context
+
+        def initialize(request_context)
+          @request_context = request_context
+        end
+
+        def paginate(relation)
+          pagination = KeysetPaginationInfo.new(relation, request_context)
+
+          paged_relation = relation.limit(per_page)
+
+          if conds = conditions(pagination)
+            paged_relation = paged_relation.where(*conds)
+          end
+
+          # In all cases: sort by primary key (possibly in addition to another sort column)
+          paged_relation = paged_relation.order(pagination.primary_key => pagination.sort)
+
+          add_default_pagination_headers
+
+          if last_record = paged_relation.last
+            next_page_params = pagination.params_for(last_record)
+            add_navigation_links(next_page_params)
+          end
+
+          paged_relation
+        end
+
+        private
+
+        def conditions(pagination)
+          fields = pagination.fields
+
+          return nil if fields.empty?
+
+          placeholder = fields.map { '?' }
+
+          comp = if pagination.sort_ascending?
+                   '>'
+                 else
+                   '<'
+                 end
+
+          [
+            # Row value comparison:
+            # (A, B) < (a, b) <=> (A < a) OR (A = a AND B < b)
+            #     <=> A <= a AND ((A < a) OR (A = a AND B < b))
+            "(#{fields.keys.join(',')}) #{comp} (#{placeholder.join(',')})",
+            *fields.values
+          ]
+        end
+
+        def per_page
+          params[:per_page]
+        end
+
+        def add_default_pagination_headers
+          header 'X-Per-Page',    per_page.to_s
+        end
+
+        def add_navigation_links(next_page_params)
+          header 'X-Next-Page', page_href(next_page_params)
+          header 'Link', link_for('next', next_page_params)
         end
 
-        relation
+        def page_href(next_page_params)
+          request_url = request.url.split('?').first
+          request_params = params.dup
+          request_params[:per_page] = per_page
+
+          request_params.merge!(next_page_params) if next_page_params
+
+          "#{request_url}?#{request_params.to_query}"
+        end
+
+        def link_for(rel, next_page_params)
+          %(<#{page_href(next_page_params)}>; rel="#{rel}")
+        end
       end
 
-      def data_without_counts?(paginated_data)
-        paginated_data.is_a?(Kaminari::PaginatableWithoutCount)
+      class DefaultPaginationStrategy
+        attr_reader :request_context
+        delegate :params, :header, :request, to: :request_context
+
+        def initialize(request_context)
+          @request_context = request_context
+        end
+
+        def paginate(relation)
+          relation = add_default_order(relation)
+
+          relation.page(params[:page]).per(params[:per_page]).tap do |data|
+            add_pagination_headers(data)
+          end
+        end
+
+        private
+
+        def add_default_order(relation)
+          if relation.is_a?(ActiveRecord::Relation) && relation.order_values.empty?
+            relation = relation.order(:id)
+          end
+
+          relation
+        end
+
+        def add_pagination_headers(paginated_data)
+          header 'X-Per-Page',    paginated_data.limit_value.to_s
+          header 'X-Page',        paginated_data.current_page.to_s
+          header 'X-Next-Page',   paginated_data.next_page.to_s
+          header 'X-Prev-Page',   paginated_data.prev_page.to_s
+          header 'Link',          pagination_links(paginated_data)
+
+          return if data_without_counts?(paginated_data)
+
+          header 'X-Total',       paginated_data.total_count.to_s
+          header 'X-Total-Pages', total_pages(paginated_data).to_s
+        end
+
+        def pagination_links(paginated_data)
+          request_url = request.url.split('?').first
+          request_params = params.clone
+          request_params[:per_page] = paginated_data.limit_value
+
+          links = []
+
+          request_params[:page] = paginated_data.prev_page
+          links << %(<#{request_url}?#{request_params.to_query}>; rel="prev") if request_params[:page]
+
+          request_params[:page] = paginated_data.next_page
+          links << %(<#{request_url}?#{request_params.to_query}>; rel="next") if request_params[:page]
+
+          request_params[:page] = 1
+          links << %(<#{request_url}?#{request_params.to_query}>; rel="first")
+
+          unless data_without_counts?(paginated_data)
+            request_params[:page] = total_pages(paginated_data)
+            links << %(<#{request_url}?#{request_params.to_query}>; rel="last")
+          end
+
+          links.join(', ')
+        end
+
+        def total_pages(paginated_data)
+          # Ensure there is in total at least 1 page
+          [paginated_data.total_pages, 1].max
+        end
+
+        def data_without_counts?(paginated_data)
+          paginated_data.is_a?(Kaminari::PaginatableWithoutCount)
+        end
       end
     end
   end