Introduce RunsPage model to handle cursor-based pagination of runs

app/controllers/maintenance_tasks/tasks_controller.rb

@@ -20,7 +20,9 @@ def index
     def show
       @task = TaskData.find(params.fetch(:id))
       set_refresh if @task.last_run&.active?
-      @pagy, @previous_runs = pagy(@task.previous_runs)
+
+      @runs_page = RunsPage.new(@task.previous_runs, params[:cursor])
+      # @pagy, @previous_runs = pagy(@task.previous_runs)
     end
 
     # Runs a given Task and redirects to the Task page.

app/models/maintenance_tasks/runs_page.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+module MaintenanceTasks
+  # This class is responsible for handling cursor-based pagination for Run
+  # records.
+  #
+  # @api private
+  class RunsPage
+    # The number of Runs to show on a single Task page.
+    RUNS_PER_PAGE = 20
+
+    # Initializes a Runs Page with a Runs relation and a cursor. This page is
+    # used by the views to render a set of Runs.
+    # @param runs [ActiveRecord::Relation<MaintenanceTasks::Run>] the relation
+    #   of Run records to be paginated.
+    # @param cursor [String, nil] the id that serves as the cursor when
+    #   querying the Runs dataset to produce a page of Runs. If nil, the first
+    #   Runs in the relation are used.
+    def initialize(runs, cursor)
+      @runs = runs
+      @cursor = cursor
+    end
+
+    # Returns the records for a Page, taking into account the cursor if one is
+    # present. Limits the number of records to 20.
+    #
+    # @return [ActiveRecord::Relation<MaintenanceTasks::Run>] a limited amount
+    #  of Run records.
+    def records
+      runs_after_cursor = if @cursor.present?
+        @runs.where('id < ?', @cursor)
+      else
+        @runs
+      end
+      runs_after_cursor.limit(RUNS_PER_PAGE)
+    end
+
+    # Returns the cursor to use for the next Page of Runs. It is the id of the
+    # last record on the current Page.
+    #
+    # @return [Integer] the id of the last record for the Page.
+    def next_cursor
+      records.last.id
+    end
+
+    # Returns whether this Page is the last one.
+    #
+    # @return [Boolean] whether this Page contains the last Run record in the
+    #   Runs dataset that is being paginated.
+    def last?
+      @runs.last == records.last
+    end
+  end
+end

app/views/maintenance_tasks/tasks/show.html.erb

@@ -41,12 +41,12 @@
   <pre><code><%= highlight_code(code) %></code></pre>
 <% end %>
 
-<% if @previous_runs.present? %>
+<% if @runs_page.records.any? %>
   <hr/>
 
   <h4 class="title is-4">Previous Runs</h4>
 
-  <%= render @previous_runs %>
+  <%= render @runs_page.records %>
 
-  <%= pagination(@pagy) %>
+  <%= link_to "Next page", task_path(@task, cursor: @runs_page.next_cursor) unless @runs_page.last? %>
 <% end %>

test/models/maintenance_tasks/runs_page_test.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+module MaintenanceTasks
+  class RunsPageTest < ActiveSupport::TestCase
+    setup do
+      @task_name = 'Maintenance::TestTask'
+      21.times do
+        Run.create!(
+          task_name: @task_name,
+          started_at: Time.now,
+          tick_count: 10,
+          tick_total: 10,
+          status: :succeeded,
+          ended_at: Time.now
+        )
+      end
+      @runs = Run.where(task_name: @task_name).order(created_at: :desc)
+    end
+
+    test '#records returns the most recent 20 runs when there is no cursor' do
+      runs_page = RunsPage.new(@runs, nil)
+      assert_equal @runs.first(20), runs_page.records
+    end
+
+    test '#records returns 20 runs after cursor if one is present' do
+      runs_page = RunsPage.new(@runs, @runs.first.id)
+      assert_equal @runs.last(20), runs_page.records
+    end
+
+    test '#next_cursor returns the id of the last run in the record set' do
+      last_id = @runs.last.id
+      runs_page = RunsPage.new(@runs, @runs.first.id)
+      assert_equal last_id, runs_page.next_cursor
+    end
+
+    test '#last? returns true if the last run in the record set is the last run for the relation' do
+      runs_page = RunsPage.new(@runs, @runs.first.id)
+      assert_predicate runs_page, :last?
+    end
+
+    test '#last? returns false if the last run in the record set is not the last run for the relation' do
+      runs_page = RunsPage.new(@runs, nil)
+      refute_predicate runs_page, :last?
+    end
+  end
+end

test/system/maintenance_tasks/tasks_test.rb

@@ -40,6 +40,38 @@ class TasksTest < ApplicationSystemTestCase
       assert_text 'Ran for less than 5 seconds, finished 8 days ago.'
     end
 
+    test 'view a Task with multiple pages of Runs' do
+      Run.create!(
+        task_name: 'Maintenance::TestTask',
+        created_at: 1.hour.ago,
+        started_at: 1.hour.ago,
+        tick_count: 2,
+        tick_total: 10,
+        status: :errored,
+        ended_at: 1.hour.ago
+      )
+      21.times do |i|
+        Run.create!(
+          task_name: 'Maintenance::TestTask',
+          created_at: i.minutes.ago,
+          started_at: i.minutes.ago,
+          tick_count: 10,
+          tick_total: 10,
+          status: :succeeded,
+          ended_at: i.minutes.ago
+        )
+      end
+
+      visit maintenance_tasks_path
+
+      click_on('Maintenance::TestTask')
+      assert_no_text 'Errored'
+
+      click_on('Next page')
+      assert_text 'Errored'
+      assert_no_link 'Next page'
+    end
+
     test 'show a deleted Task' do
       visit maintenance_tasks_path + '/tasks/Maintenance::DeletedTask'