Fix SetDefaultIterationCadences migration

- fix Group class inside the migration
- add specs checking the correct class is being called
- reschedule the migration

changelogs/unreleased/fix-iteration-migr.yml

+---
+title: Fix setting default cadences migration
+merge_request: 54598
+author:
+type: other

db/post_migrate/20201231133921_schedule_set_default_iteration_cadences.rb

@@ -17,11 +17,7 @@ class ScheduleSetDefaultIterationCadences < ActiveRecord::Migration[6.0]
   disable_ddl_transaction!
 
   def up
-    Iteration.select(:group_id).distinct.each_batch(of: BATCH_SIZE, column: :group_id) do |batch, index|
-      group_ids = batch.pluck(:group_id)
-
-      migrate_in(index * DELAY_INTERVAL, MIGRATION_CLASS, group_ids)
-    end
+    # Do nothing, rescheduling migration: 20210219102900_reschedule_set_default_iteration_cadences.rb
   end
 
   def down

db/post_migrate/20210219102900_reschedule_set_default_iteration_cadences.rb

+# frozen_string_literal: true
+
+class RescheduleSetDefaultIterationCadences < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 1_000
+  DELAY_INTERVAL = 2.minutes.to_i
+  MIGRATION_CLASS = 'SetDefaultIterationCadences'
+
+  class Iteration < ActiveRecord::Base # rubocop:disable Style/Documentation
+    include EachBatch
+
+    self.table_name = 'sprints'
+  end
+
+  disable_ddl_transaction!
+
+  def up
+    Iteration.select(:group_id).distinct.each_batch(of: BATCH_SIZE, column: :group_id) do |batch, index|
+      group_ids = batch.pluck(:group_id)
+
+      migrate_in(index * DELAY_INTERVAL, MIGRATION_CLASS, group_ids)
+    end
+  end
+
+  def down
+    # Not needed
+  end
+end

db/schema_migrations/20210219102900

+42e06332b279aaac7044243df0a8bd5525025db7d8c22bc474c0874e85f525f5
\ No newline at end of file

lib/gitlab/background_migration/set_default_iteration_cadences.rb

@@ -16,6 +16,8 @@ module Gitlab
 
       class Group < ApplicationRecord
         self.table_name = 'namespaces'
+
+        self.inheritance_column = :_type_disabled
       end
 
       def perform(*group_ids)
@@ -27,6 +29,7 @@ module Gitlab
 
       def create_iterations_cadences(group_ids)
         groups_with_cadence = IterationCadence.select(:group_id)
+
         new_cadences = Group.where(id: group_ids).where.not(id: groups_with_cadence).map do |group|
           last_iteration = Iteration.where(group_id: group.id).order(:start_date)&.last
 
@@ -44,7 +47,7 @@ module Gitlab
           )
         end
 
-        IterationCadence.bulk_insert!(new_cadences.compact)
+        IterationCadence.bulk_insert!(new_cadences.compact, skip_duplicates: true)
       end
 
       def assign_iterations_cadences(group_ids)

spec/lib/gitlab/background_migration/set_default_iteration_cadences_spec.rb

@@ -17,8 +17,10 @@ RSpec.describe Gitlab::BackgroundMigration::SetDefaultIterationCadences, schema:
       let!(:iteration_2) { iterations.create!(group_id: group_3.id, iid: 1, title: 'Iteration 2', start_date: 10.days.ago, due_date: 8.days.ago) }
       let!(:iteration_3) { iterations.create!(group_id: group_3.id, iid: 1, title: 'Iteration 3', start_date: 5.days.ago, due_date: 2.days.ago) }
 
+      subject { described_class.new.perform(group_1.id, group_2.id, group_3.id, namespaces.last.id + 1) }
+
       before do
-        described_class.new.perform(group_1.id, group_2.id, group_3.id, namespaces.last.id + 1)
+        subject
       end
 
       it 'creates iterations_cadence records for the requested groups' do
@@ -45,6 +47,12 @@ RSpec.describe Gitlab::BackgroundMigration::SetDefaultIterationCadences, schema:
         expect(iteration_records.first.id).to eq(iteration_2.id)
         expect(iteration_records.second.id).to eq(iteration_3.id)
       end
+
+      it 'does not call Group class' do
+        expect(::Group).not_to receive(:where)
+
+        subject
+      end
     end
 
     context 'when an iteration cadence exists for a group' do

spec/migrations/schedule_set_default_iteration_cadences_spec.rb → spec/migrations/reschedule_set_default_iteration_cadences_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 require_migration!
 
-RSpec.describe ScheduleSetDefaultIterationCadences do
+RSpec.describe RescheduleSetDefaultIterationCadences do
   let(:namespaces) { table(:namespaces) }
   let(:iterations) { table(:sprints) }