Remove orphaned routes

This removes all orphaned project and namespace routes from the "routes"
table. This is in as a preparation step for removing dynamic route
generation, as discussed in
https://gitlab.com/gitlab-org/gitlab-ce/issues/44390.

db/migrate/20180702124358_remove_orphaned_routes.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveOrphanedRoutes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class Route < ActiveRecord::Base
+    self.table_name = 'routes'
+    include EachBatch
+
+    def self.orphaned_namespace_routes
+      where(source_type: 'Namespace')
+        .where('NOT EXISTS ( SELECT 1 FROM namespaces WHERE namespaces.id = routes.source_id )')
+    end
+
+    def self.orphaned_project_routes
+      where(source_type: 'Project')
+        .where('NOT EXISTS ( SELECT 1 FROM projects WHERE projects.id = routes.source_id )')
+    end
+  end
+
+  def up
+    # Some of these queries can take up to 10 seconds to run on GitLab.com,
+    # which is pretty close to our 15 second statement timeout. To ensure a
+    # smooth deployment procedure we disable the statement timeouts for this
+    # migration, just in case.
+    disable_statement_timeout
+
+    # On GitLab.com there are around 4000 orphaned project routes, and around
+    # 150 orphaned namespace routes.
+    [
+      Route.orphaned_project_routes,
+      Route.orphaned_namespace_routes
+    ].each do |relation|
+      relation.each_batch(of: 1_000) do |batch|
+        batch.delete_all
+      end
+    end
+  end
+
+  def down
+    # There is no way to restore orphaned routes, and this doesn't make any
+    # sense anyway.
+  end
+end