Enable mapping to nil in enums

Enum in Rails 5 does not map nil values - IOW nil value remains nil,
even if there is a key with nil value in the enum definition.

This commit overrides the underlying Enum methods so nil value is
still mapped. This solution is far from being ideal: it uses dynamic
definition of methods which introduces more magic/confusion
into the codebase.

It would be better to get rid of the nil value in enums.

app/models/ci/pipeline.rb

@@ -8,6 +8,7 @@ module Ci
     include Gitlab::OptimisticLocking
     include Gitlab::Utils::StrongMemoize
     include AtomicInternalId
+    include EnumWithNil
 
     belongs_to :project, inverse_of: :pipelines
     belongs_to :user
@@ -54,7 +55,7 @@ module Ci
 
     after_create :keep_around_commits, unless: :importing?
 
-    enum source: {
+    enum_with_nil source: {
       unknown: nil,
       push: 1,
       web: 2,
@@ -64,7 +65,7 @@ module Ci
       external: 6
     }
 
-    enum config_source: {
+    enum_with_nil config_source: {
       unknown_source: nil,
       repository_source: 1,
       auto_devops_source: 2
@@ -599,17 +600,6 @@ module Ci
       @latest_builds_with_artifacts ||= builds.latest.with_artifacts_archive.to_a
     end
 
-    # Rails 5.0 autogenerated question mark enum methods return wrong result if enum value is nil.
-    # They always return `false`.
-    # These methods overwrite autogenerated ones to return correct results.
-    def unknown?
-      Gitlab.rails5? ? source.nil? : super
-    end
-
-    def unknown_source?
-      Gitlab.rails5? ? config_source.nil? : super
-    end
-
     private
 
     def ci_yaml_from_repo

app/models/commit_status.rb

@@ -3,6 +3,7 @@ class CommitStatus < ActiveRecord::Base
   include Importable
   include AfterCommitQueue
   include Presentable
+  include EnumWithNil
 
   self.table_name = 'ci_builds'
 
@@ -39,7 +40,7 @@ class CommitStatus < ActiveRecord::Base
   scope :retried_ordered, -> { retried.ordered.includes(project: :namespace) }
   scope :after_stage, -> (index) { where('stage_idx > ?', index) }
 
-  enum failure_reason: {
+  enum_with_nil failure_reason: {
     unknown_failure: nil,
     script_failure: 1,
     api_failure: 2,
@@ -190,11 +191,4 @@ class CommitStatus < ActiveRecord::Base
       v =~ /\d+/ ? v.to_i : v
     end
   end
-
-  # Rails 5.0 autogenerated question mark enum methods return wrong result if enum value is nil.
-  # They always return `false`.
-  # This method overwrites the autogenerated one to return correct result.
-  def unknown_failure?
-    Gitlab.rails5? ? failure_reason.nil? : super
-  end
 end

app/models/concerns/enum_with_nil.rb

@@ -0,0 +1,33 @@
+module EnumWithNil
+  extend ActiveSupport::Concern
+
+  included do
+    def self.enum_with_nil(definitions)
+      # use original `enum` to auto-define all methods
+      enum(definitions)
+
+      # override auto-defined methods only for the
+      # key which uses nil value
+      definitions.each do |name, values|
+        next unless key_with_nil = values.key(nil)
+
+        # E.g. for enum_with_nil failure_reason: { unknown_failure: nil }
+        # this overrides auto-generated method `unknown_failure?`
+        define_method("#{key_with_nil}?") do
+          Gitlab.rails5? ? self[name].nil? : super()
+        end
+
+        # E.g. for enum_with_nil failure_reason: { unknown_failure: nil }
+        # this overrides auto-generated method `failure_reason`
+        define_method(name) do
+          orig = super()
+
+          return orig unless Gitlab.rails5?
+          return orig unless orig.nil?
+
+          self.class.public_send(name.to_s.pluralize).key(nil) # rubocop:disable GitlabSecurity/PublicSend
+        end
+      end
+    end
+  end
+end