Don't delete assigned MRs/issues when user is deleted

app/models/user.rb

@@ -83,8 +83,6 @@ class User < ActiveRecord::Base
   has_many :events,                   dependent: :destroy, foreign_key: :author_id
   has_many :subscriptions,            dependent: :destroy
   has_many :recent_events, -> { order "id DESC" }, foreign_key: :author_id,   class_name: "Event"
-  has_many :assigned_issues,          dependent: :destroy, foreign_key: :assignee_id, class_name: "Issue"
-  has_many :assigned_merge_requests,  dependent: :destroy, foreign_key: :assignee_id, class_name: "MergeRequest"
   has_many :oauth_applications, class_name: 'Doorkeeper::Application', as: :owner, dependent: :destroy
   has_one  :abuse_report,             dependent: :destroy
   has_many :spam_logs,                dependent: :destroy
@@ -94,6 +92,9 @@ class User < ActiveRecord::Base
   has_many :notification_settings,    dependent: :destroy
   has_many :award_emoji,              dependent: :destroy
 
+  has_many :assigned_issues,          dependent: :nullify, foreign_key: :assignee_id, class_name: "Issue"
+  has_many :assigned_merge_requests,  dependent: :nullify, foreign_key: :assignee_id, class_name: "MergeRequest"
+
   #
   # Validations
   #

changelogs/unreleased/dont-delete-assigned-issuables.yml

+---
+title: Don't delete assigned MRs/issues when user is deleted
+merge_request:
+author: