Create migration to remove existing cyclic epics

We have code to prevent this but there are existing
rows that needs to be cleaned up

ee/db/migrate/20190327085945_remove_cyclic_hierarchies_in_epics.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveCyclicHierarchiesInEpics < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    epics_in_loops_sql = <<~SQL
+      WITH RECURSIVE base_and_descendants (id, path, cycle) AS (
+        SELECT epics.id, ARRAY[epics.id], false FROM epics WHERE epics.parent_id IS NOT NULL
+      UNION
+          SELECT epics.id, path || epics.id, epics.id = ANY(path)
+          FROM epics, base_and_descendants
+          WHERE epics.parent_id = base_and_descendants.id AND NOT cycle
+      )
+      SELECT id, array_to_string(path, ',') AS path FROM base_and_descendants WHERE cycle ORDER BY id
+    SQL
+
+    # Group by sorted path so we can group epics that belong to the same loop
+    epics_grouped_by_loop = select_all(epics_in_loops_sql).group_by { |r| r['path'].split(',').uniq.sort.join(',') }
+
+    # We only need to update the first epic of the loop to break the cycle
+    epic_ids_to_update = epics_grouped_by_loop.map { |path, epics| epics.first['id'] }
+
+    update_column_in_batches(:epics, :parent_id, nil) do |table, query|
+      query.where(
+        table[:id].in(epic_ids_to_update)
+      )
+    end
+  end
+
+  def down
+  end
+end

ee/spec/migrations/remove_cyclic_hierarchies_in_epics_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('ee', 'db', 'migrate', '20190327085945_remove_cyclic_hierarchies_in_epics.rb')
+
+describe RemoveCyclicHierarchiesInEpics, :migration, :postgresql do
+  let(:epics) { table(:epics) }
+  let(:group) { table(:namespaces).create(name: 'Group 1', type: 'Group', path: 'group_1') }
+  let(:user) { table(:users).create!(email: 'email@email.com', name: 'foo', username: 'foo', projects_limit: 0) }
+
+  def create_epic_with_defaults!(attributes = {})
+    @last_iid += 1
+
+    epics.create!({iid: @last_iid, group_id: group.id, author_id: user.id, title: "Epic", title_html: "Epic"}.merge(attributes))
+  end
+
+  let!(:epic_self_loop) { create_epic_with_defaults! }
+  let!(:epic_loop_1) { create_epic_with_defaults! }
+  let!(:epic_loop_2) { create_epic_with_defaults! }
+
+  let!(:epic_not_in_loop) { create_epic_with_defaults! }
+
+  before(:all) do
+    @last_iid = 0
+  end
+
+  before do
+    epic_self_loop.update(parent_id: epic_self_loop.id)
+
+    epic_loop_1_1 = create_epic_with_defaults!(parent_id: epic_loop_1.id)
+    epic_loop_1.update(parent_id: epic_loop_1_1.id)
+
+    epic_loop_2_1 = create_epic_with_defaults!(parent_id: epic_loop_2.id)
+    epic_loop_2_2 = create_epic_with_defaults!(parent_id: epic_loop_2_1.id)
+    epic_loop_2_3 = create_epic_with_defaults!(parent_id: epic_loop_2_2.id)
+    epic_loop_2.update(parent_id: epic_loop_2_3.id)
+
+    epic_parent = create_epic_with_defaults!
+    epic_not_in_loop.update(parent_id: epic_parent.id)
+  end
+
+  it 'clears parent_id of epic in loop' do
+    migrate!
+
+    expect(epic_self_loop.reload.parent_id).to be_nil
+    expect(epic_loop_1.reload.parent_id).to be_nil
+    expect(epic_loop_2.reload.parent_id).to be_nil
+  end
+
+  it 'does not clear parent_id for other epics' do
+    migrate!
+
+    expect(epic_not_in_loop.reload.parent_id).not_to be_nil
+  end
+end
+
+