Exposing past feed experiments in /admin/abtests (#17818)

Prior to this commit, as a matter of undocumented practice, we would
overwrite past experiments with upcoming experiments.

With this commit, we're leveraging a feature of the field_test gem:
declaring a winner.  Instead of overwriting an experiment with the next
experiment, we're going to declare the expiring experiment's winner then
prepend to the ./config/field_test.yml the new experiment.

The [field_test/app/views/field_test/experiments/index.html.erb][1] then
uses logic to first render "active" experiments and "completed"
experiments.  A "completed" experiment is one in which we've declared a
winner.

Closes forem/forem#17816

[1]:https://github.com/ankane/field_test/blob/master/app/views/field_test/experiments/index.html.erb

app/models/ab_experiment.rb

@@ -14,8 +14,10 @@ class AbExperiment < SimpleDelegator
   # @see ./config/feed-variants/README.md
   ORIGINAL_VARIANT = "original".freeze
 
-  CURRENT_FEED_STRATEGY_EXPERIMENT = FieldTest.config["experiments"]&.keys
-  &.detect { |e| e.start_with? "feed_strategy" }.freeze
+  # @note a present assumption is that we will have a feed_strategy oriented experiment.
+  CURRENT_FEED_STRATEGY_EXPERIMENT = FieldTest.config["experiments"].select do |key, value|
+    key.start_with?("feed_strategy") && !value["winner"]
+  end.keys.first
 
   # Sometimes we might want to repurpose the same AbExperiment logic
   # for different experiments.  This provides the tooling for that

config/field_test.yml

@@ -26,9 +26,77 @@ experiments:
       - user_views_pages_on_at_least_nine_different_days_within_two_weeks
       - user_publishes_post_at_least_two_times_within_week
       - user_publishes_post_at_least_two_times_within_two_weeks
-exclude:
-  bots: true
-
+  feed_strategy_starting_20220518:
+    winner: 20220518-variant
+    # NOTE: Required as we want only want to consider for conversion events that
+    #       occurred on or after the given start_date.
+    start_date: 2022-05-09
+    variants:
+      - 20220422-variant
+      - 20220518-variant
+    weights:
+      - 80
+      - 20
+    goals:
+      - user_creates_pageview
+      - user_creates_article_reaction
+      - user_creates_comment
+      - user_publishes_post
+      - user_views_pages_on_at_least_four_different_days_within_a_week
+      - user_creates_article_reaction_on_four_different_days_within_a_week
+      - user_creates_comment_on_at_least_four_different_days_within_a_week
+      - user_publishes_post_on_four_different_days_within_a_week
+      # These are historical and may be userful
+      - user_views_pages_on_at_least_four_different_hours_within_a_day
+      - user_views_pages_on_at_least_twelve_different_hours_within_five_days
+      - user_views_pages_on_at_least_nine_different_days_within_two_weeks
+      - user_publishes_post_at_least_two_times_within_week
+  feed_strategy_starting_20220509:
+    winner: 20220422-variant
+    # NOTE: Required as we want only want to consider for conversion events that
+    #       occurred on or after the given start_date.
+    start_date: 2022-05-09
+    variants:
+      - 20220422-variant
+      - 20220509-variant
+    weights:
+      - 80
+      - 20
+    goals:
+      - user_creates_pageview
+      - user_creates_article_reaction
+      - user_creates_comment
+      - user_publishes_post
+      - user_views_pages_on_at_least_four_different_days_within_a_week
+      - user_creates_article_reaction_on_four_different_days_within_a_week
+      - user_creates_comment_on_at_least_four_different_days_within_a_week
+      - user_publishes_post_on_four_different_days_within_a_week
+      # These are historical and may be userful
+      - user_views_pages_on_at_least_four_different_hours_within_a_day
+      - user_views_pages_on_at_least_twelve_different_hours_within_five_days
+      - user_views_pages_on_at_least_nine_different_days_within_two_weeks
+      - user_publishes_post_at_least_two_times_within_week
+  feed_strategy_starting_20220422:
+    winner: 20220422-variant
+    # NOTE: Required as we want only want to consider for conversion events that
+    #       occurred on or after the given start_date.
+    start_date: 2022-04-22
+    variants:
+      - 20220415-incumbent
+      - 20220422-jennie-variant
+    weights:
+      - 80
+      - 20
+    goals:
+      - user_creates_comment
+      - user_creates_comment_on_at_least_four_different_days_within_a_week
+      - user_views_pages_on_at_least_four_different_days_within_a_week
+      - user_views_pages_on_at_least_four_different_hours_within_a_day
+      - user_views_pages_on_at_least_nine_different_days_within_two_weeks
+      - user_views_pages_on_at_least_twelve_different_hours_within_five_days
+      - user_publishes_post
+      - user_publishes_post_at_least_two_times_within_week
+      - user_publishes_post_at_least_two_times_within_two_weeks
 # Dashboard performance
 cache: true
 precision: 2

spec/models/ab_experiment_spec.rb

@@ -4,6 +4,27 @@ RSpec.describe AbExperiment do
   let(:controller) { ApplicationController.new }
   let(:user) { double }
 
+  describe "CURRENT_FEED_STRATEGY_EXPERIMENT" do
+    subject(:variant) { described_class::CURRENT_FEED_STRATEGY_EXPERIMENT }
+
+    it { is_expected.to be_a String }
+
+    it "is a key in the FieldTest.config['experiments']" do
+      expect(FieldTest.config["experiments"].keys).to include(variant)
+    end
+  end
+
+  describe "validate field test experiment configuration" do
+    it "only allow at most one active feed_strategy (e.g. an experiment without a winner)" do
+      field_tests = Psych.load(Rails.root.join("config/field_test.yml").read)
+      active_field_tests = field_tests.fetch("experiments", {})
+        .select { |key, values| key.start_with?("feed_strategy") && !values["winner"] }
+
+      # We are only going to allow
+      expect(active_field_tests.size).to be <= 1
+    end
+  end
+
   describe ".register_conversions_for" do
     it "forwards delegates to Converter.call" do
       allow(described_class::GoalConversionHandler).to receive(:call)