Address some feedback from last code review

app/models/gpg_signature.rb

@@ -23,8 +23,13 @@ class GpgSignature < ActiveRecord::Base
   def gpg_key=(model)
     case model
-    when GpgKey       then super
-    when GpgKeySubkey then self.gpg_key_subkey = model
+    when GpgKey
+      super
+    when GpgKeySubkey
+      self.gpg_key_subkey = model
+    when NilClass
+      super
+      self.gpg_key_subkey = nil
     end
   end

db/migrate/20170927161718_create_gpg_key_subkeys.rb

@@ -3,11 +3,11 @@ class CreateGpgKeySubkeys < ActiveRecord::Migration
   def up
     create_table :gpg_key_subkeys do |t|
+      t.references :gpg_key, null: false, index: true, foreign_key: { on_delete: :cascade }
+
       t.binary :keyid
       t.binary :fingerprint
-      t.references :gpg_key, null: false, index: true, foreign_key: { on_delete: :cascade }
-
       t.index :keyid, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
       t.index :fingerprint, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
     end

db/post_migrate/20171002161539_create_gpg_key_subkeys_for_existing_gpg_keys.rb

@@ -28,8 +28,10 @@ class CreateGpgKeySubkeysForExistingGpgKeys < ActiveRecord::Migration
   end
   def up
-    GpgKey.each_batch do |batch|
+    GpgKey.with_subkeys.each_batch do |batch|
       batch.each do |gpg_key|
+        return if gpg_key.subkeys.any?
+
         create_subkeys(gpg_key) && update_signatures(gpg_key)
       end
     end

db/schema.rb

@@ -580,9 +580,9 @@ ActiveRecord::Schema.define(version: 20171004121444) do
   add_index "forked_project_links", ["forked_to_project_id"], name: "index_forked_project_links_on_forked_to_project_id", unique: true, using: :btree
   create_table "gpg_key_subkeys", force: :cascade do |t|
+    t.integer "gpg_key_id", null: false
     t.binary "keyid"
     t.binary "fingerprint"
-    t.integer "gpg_key_id", null: false
   end
   add_index "gpg_key_subkeys", ["fingerprint"], name: "index_gpg_key_subkeys_on_fingerprint", unique: true, using: :btree

lib/gitlab/gpg/commit.rb

@@ -43,6 +43,8 @@ module Gitlab
           # key belonging to the keyid.
           # This way we can add the key to the temporary keychain and extract
           # the proper signature.
+          # NOTE: the invoked method is #fingerprint but it's only returning
+          # 16 characters (the format used by keyid) instead of 40.
           gpg_key = find_gpg_key(verified_signature.fingerprint)
           if gpg_key