Add keyset pagination for API calls.

Closes #45756.

lib/api/helpers/pagination.rb

@@ -2,15 +2,175 @@ module API
   module Helpers
     module Pagination
       def paginate(relation)
-        DefaultPaginationStrategy.new(self).paginate(relation)
+        strategy = if params[:pagination] == 'keyset'
+                     KeysetPaginationStrategy
+                   else
+                     DefaultPaginationStrategy
+                   end
+
+        strategy.new(self).paginate(relation)
+      end
+
+      class KeysetPaginationInfo
+        attr_reader :relation, :request_context
+
+        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
+
+          @relation = relation
+          @request_context = request_context
+        end
+
+        def fields
+          keys.zip(values).reject { |_, v| v.nil? }.to_h
+        end
+
+        def column_for_order_by(relation)
+          relation.order_values.first&.expr&.name
+        end
+
+        # 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
+
+        # Do we only sort by primary key?
+        def order_by_primary_key?
+          keys.size == 1 && keys.first == primary_key
+        end
+
+        def primary_key
+          relation.model.primary_key.to_sym
+        end
+
+        def sort_ascending?
+          sort == :asc
+        end
+
+        # 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
+
+        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
+
+        # 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
+
+      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
+
+        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
 
       class DefaultPaginationStrategy
-        attr_reader :ctx
-        delegate :params, :header, :request, to: :ctx
+        attr_reader :request_context
+        delegate :params, :header, :request, to: :request_context
 
-        def initialize(ctx)
-          @ctx = ctx
+        def initialize(request_context)
+          @request_context = request_context
         end
 
         def paginate(relation)

spec/lib/api/helpers/pagination_spec.rb

@@ -7,7 +7,203 @@ describe API::Helpers::Pagination do
     Class.new.include(described_class).new
   end
 
-  describe '#paginate' do
+  describe '#paginate (keyset pagination)' do
+    let(:value) { spy('return value') }
+
+    before do
+      allow(value).to receive(:to_query).and_return(value)
+
+      allow(subject).to receive(:header).and_return(value)
+      allow(subject).to receive(:params).and_return(value)
+      allow(subject).to receive(:request).and_return(value)
+    end
+
+    context 'when resource can be paginated' do
+      let!(:projects) do
+        [
+          create(:project, name: 'One'),
+          create(:project, name: 'Two'),
+          create(:project, name: 'Three')
+        ].sort_by { |e| -e.id } # sort by id desc (this is the default sort order for the API)
+      end
+
+      describe 'first page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ pagination: 'keyset', per_page: 2 })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 2
+        end
+
+        it 'returns the first two records (by id desc)' do
+          expect(subject.paginate(resource)).to eq(projects[0..1])
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Per-Page', '2')
+          expect_header('X-Next-Page', "#{value}?ks_prev_id=#{projects[1].id}&pagination=keyset&per_page=2")
+
+          expect_header('Link', anything) do |_key, val|
+            expect(val).to include('rel="next"')
+          end
+
+          subject.paginate(resource)
+        end
+      end
+
+      describe 'second page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ pagination: 'keyset', per_page: 2, ks_prev_id: projects[1].id })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 1
+        end
+
+        it 'returns the third record' do
+          expect(subject.paginate(resource)).to eq(projects[2..2])
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Per-Page', '2')
+          expect_header('X-Next-Page', "#{value}?ks_prev_id=#{projects[2].id}&pagination=keyset&per_page=2")
+
+          expect_header('Link', anything) do |_key, val|
+            expect(val).to include('rel="next"')
+          end
+
+          subject.paginate(resource)
+        end
+      end
+
+      describe 'third page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ pagination: 'keyset', per_page: 2, ks_prev_id: projects[2].id })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 0
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Per-Page', '2')
+          expect(subject).not_to receive(:header).with('Link')
+
+          subject.paginate(resource)
+        end
+      end
+
+      context 'if order' do
+        context 'is not present' do
+          before do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', per_page: 2 })
+          end
+
+          it 'is not present it adds default order(:id) desc' do
+            resource.order_values = []
+
+            paginated_relation = subject.paginate(resource)
+
+            expect(resource.order_values).to be_empty
+            expect(paginated_relation.order_values).to be_present
+            expect(paginated_relation.order_values.size).to eq(1)
+            expect(paginated_relation.order_values.first).to be_descending
+            expect(paginated_relation.order_values.first.expr.name).to eq :id
+          end
+        end
+
+        context 'is present' do
+          let(:resource) { Project.all.order(name: :desc) }
+          let!(:projects) do
+            [
+              create(:project, name: 'One'),
+              create(:project, name: 'Two'),
+              create(:project, name: 'Three'),
+              create(:project, name: 'Three'), # Note the duplicate name
+              create(:project, name: 'Four'),
+              create(:project, name: 'Five'),
+              create(:project, name: 'Six')
+            ]
+
+            # if we sort this by name descending, id descending, this yields:
+            # {
+            #   2 => "Two",
+            #   4 => "Three",
+            #   3 => "Three",
+            #   7 => "Six",
+            #   1 => "One",
+            #   5 => "Four",
+            #   6 => "Five"
+            # }
+            #
+            # (key is the id)
+          end
+
+          it 'it also orders by primary key' do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', per_page: 2 })
+            paginated_relation = subject.paginate(resource)
+
+            expect(paginated_relation.order_values).to be_present
+            expect(paginated_relation.order_values.size).to eq(2)
+            expect(paginated_relation.order_values.first).to be_descending
+            expect(paginated_relation.order_values.first.expr.name).to eq :name
+            expect(paginated_relation.order_values.second).to be_descending
+            expect(paginated_relation.order_values.second.expr.name).to eq :id
+          end
+
+          it 'it returns the right records (first page)' do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', per_page: 2 })
+            result = subject.paginate(resource)
+
+            expect(result.first).to eq(projects[1])
+            expect(result.second).to eq(projects[3])
+          end
+
+          it 'it returns the right records (second page)' do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', ks_prev_id: projects[3].id, ks_prev_name: projects[3].name, per_page: 2 })
+            result = subject.paginate(resource)
+
+            expect(result.first).to eq(projects[2])
+            expect(result.second).to eq(projects[6])
+          end
+
+          it 'it returns the right records (third page), note increased per_page' do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', ks_prev_id: projects[6].id, ks_prev_name: projects[6].name, per_page: 5 })
+            result = subject.paginate(resource)
+
+            expect(result.size).to eq(3)
+            expect(result.first).to eq(projects[0])
+            expect(result.second).to eq(projects[4])
+            expect(result.last).to eq(projects[5])
+          end
+
+          it 'it returns the right link to the next page' do
+            allow(subject).to receive(:params)
+              .and_return({ pagination: 'keyset', ks_prev_id: projects[3].id, ks_prev_name: projects[3].name, per_page: 2 })
+            expect_header('X-Per-Page', '2')
+            expect_header('X-Next-Page', "#{value}?ks_prev_id=#{projects[6].id}&ks_prev_name=#{projects[6].name}&pagination=keyset&per_page=2")
+
+            expect_header('Link', anything) do |_key, val|
+              expect(val).to include('rel="next"')
+            end
+
+            subject.paginate(resource)
+          end
+        end
+      end
+    end
+  end
+
+  describe '#paginate (default offset-based pagination)' do
     let(:value) { spy('return value') }
 
     before do
@@ -146,14 +342,14 @@ describe API::Helpers::Pagination do
         end
       end
     end
+  end
 
-    def expect_header(*args, &block)
-      expect(subject).to receive(:header).with(*args, &block)
-    end
+  def expect_header(*args, &block)
+    expect(subject).to receive(:header).with(*args, &block)
+  end
 
-    def expect_message(method)
-      expect(subject).to receive(method)
-        .at_least(:once).and_return(value)
-    end
+  def expect_message(method)
+    expect(subject).to receive(method)
+      .at_least(:once).and_return(value)
   end
 end