use db's on_delete instead of has_many :dependent

app/models/gpg_key.rb

@@ -7,7 +7,7 @@ class GpgKey < ActiveRecord::Base
   sha_attribute :fingerprint
 
   belongs_to :user
-  has_many :gpg_signatures, dependent: :nullify
+  has_many :gpg_signatures
 
   validates :user, presence: true
 

app/models/user.rb

@@ -76,7 +76,7 @@ class User < ActiveRecord::Base
     where(type.not_eq('DeployKey').or(type.eq(nil)))
   end, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :deploy_keys, -> { where(type: 'DeployKey') }, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
-  has_many :gpg_keys, dependent: :destroy
+  has_many :gpg_keys
 
   has_many :emails, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :personal_access_tokens, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent

db/migrate/20170222111732_create_gpg_keys.rb

@@ -5,7 +5,7 @@ class CreateGpgKeys < ActiveRecord::Migration
     create_table :gpg_keys do |t|
       t.timestamps_with_timezone null: false
 
-      t.references :user, index: true, foreign_key: true
+      t.references :user, index: true, foreign_key: { on_delete: :cascade }
 
       t.binary :primary_keyid, limit: Gitlab::Database.mysql? ? 20 : nil
       t.binary :fingerprint, limit: Gitlab::Database.mysql? ? 20 : nil

db/migrate/20170613154149_create_gpg_signatures.rb

@@ -5,8 +5,8 @@ class CreateGpgSignatures < ActiveRecord::Migration
     create_table :gpg_signatures do |t|
       t.timestamps_with_timezone null: false
 
-      t.references :project, index: true, foreign_key: true
-      t.references :gpg_key, index: true, foreign_key: true
+      t.references :project, index: true, foreign_key: { on_delete: :cascade }
+      t.references :gpg_key, index: true, foreign_key: { on_delete: :nullify }
 
       t.boolean :valid_signature
 

db/schema.rb

@@ -1631,9 +1631,9 @@ ActiveRecord::Schema.define(version: 20170725145659) do
   add_foreign_key "environments", "projects", name: "fk_d1c8c1da6a", on_delete: :cascade
   add_foreign_key "events", "projects", name: "fk_0434b48643", on_delete: :cascade
   add_foreign_key "forked_project_links", "projects", column: "forked_to_project_id", name: "fk_434510edb0", on_delete: :cascade
-  add_foreign_key "gpg_keys", "users"
-  add_foreign_key "gpg_signatures", "gpg_keys"
-  add_foreign_key "gpg_signatures", "projects"
+  add_foreign_key "gpg_keys", "users", on_delete: :cascade
+  add_foreign_key "gpg_signatures", "gpg_keys", on_delete: :nullify
+  add_foreign_key "gpg_signatures", "projects", on_delete: :cascade
   add_foreign_key "issue_assignees", "issues", name: "fk_b7d881734a", on_delete: :cascade
   add_foreign_key "issue_assignees", "users", name: "fk_5e0c8d9154", on_delete: :cascade
   add_foreign_key "issue_metrics", "issues", on_delete: :cascade