Add oldest_authored_at scope to Version

Replicate `Version#authored_at` logic in a SQL case statement and call
SQL `minimum` on that to get the oldest `authored_at`

Add tests for the model using similar structure as tests for `authored_at`

app/controllers/versions_controller.rb

@@ -3,7 +3,7 @@ class VersionsController < ApplicationController
 
   def index
     set_page
-    set_oldest_version_date
+    @oldest_version_date = @rubygem.versions.oldest_authored_at
     @versions = @rubygem.versions.by_position.page(@page).per(Gemcutter::VERSIONS_PER_PAGE)
   end
 
@@ -15,12 +15,4 @@ def show
     @on_version_page = true
     render "rubygems/show"
   end
-
-  private
-
-  def set_oldest_version_date
-    oldest_created_at = @rubygem.versions.order(:created_at).first
-    oldest_built_at = @rubygem.versions.order(:built_at).first
-    @oldest_version_date = [oldest_created_at, oldest_built_at].compact.map(&:authored_at).min
-  end
 end

app/models/version.rb

@@ -53,6 +53,19 @@ class Version < ApplicationRecord # rubocop:disable Metrics/ClassLength
   validate :metadata_attribute_length
   validate :no_dashes_in_version_number, on: :create
 
+  scope :oldest_authored_at, lambda {
+    minimum(
+      <<~SQL.squish
+        CASE WHEN DATE(created_at) = '#{RUBYGEMS_IMPORT_DATE}'
+            AND built_at <= created_at THEN
+            built_at
+        ELSE
+            created_at
+        END
+      SQL
+    )&.in_time_zone
+  }
+
   class AuthorType < ActiveModel::Type::String
     def cast_value(value)
       if value.is_a?(Array)

test/models/version_test.rb

@@ -242,6 +242,25 @@ class VersionTest < ActiveSupport::TestCase
       end
     end
 
+    context ".oldest_authored_at scope" do
+      setup do
+        @built_at = Version::RUBYGEMS_IMPORT_DATE - 60.days
+        @version.update(built_at: @built_at, created_at: Version::RUBYGEMS_IMPORT_DATE)
+        newer_version = create(:version, rubygem: @version.rubygem)
+      end
+
+      should "return the built_at of the oldest version when created_at is the same as RUBYGEMS_IMPORT_DATE" do
+        assert_equal @built_at, Version.oldest_authored_at
+      end
+
+      should "return the created_at of the oldest version when created_at is not the same as RUBYGEMS_IMPORT_DATE" do
+        created_at = Version::RUBYGEMS_IMPORT_DATE + 1.day
+        @version.update(created_at: created_at)
+
+        assert_equal created_at, Version.oldest_authored_at
+      end
+    end
+
     should "have a rubygems version" do
       @version.update(required_rubygems_version: @required_rubygems_version)
       new_version = Version.find(@version.id)