# frozen_string_literal: true

#
# Copyright (C) 2013 - present Instructure, Inc.
#
# This file is part of Canvas.
#
# Canvas is free software: you can redistribute it and/or modify it under
# the terms of the GNU Affero General Public License as published by the Free
# Software Foundation, version 3 of the License.
#
# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
# details.
#
# You should have received a copy of the GNU Affero General Public License along
# with this program. If not, see <http://www.gnu.org/licenses/>.

class UserMerge

  def self.from(user)
    new(user)
  end

  attr_reader :from_user
  attr_accessor :target_user, :data, :merge_data

  def initialize(from_user)
    @from_user = from_user
    @target_user = nil
    @merge_data = nil
    @data = []
  end

  def into(target_user, merger: nil, source: nil)
    return unless target_user
    return if target_user == from_user
    raise 'cannot merge a test student' if from_user.preferences[:fake_student] || target_user.preferences[:fake_student]
    @target_user = target_user
    target_user.associate_with_shard(from_user.shard, :shadow)
    # we also store records for the from_user on the target shard for a split
    from_user.associate_with_shard(target_user.shard, :shadow)
    target_user.shard.activate do
      @merge_data = UserMergeData.create!(user: target_user, from_user: from_user, workflow_state: 'merging')
      @merge_data.items.create!(user: target_user, item_type: 'logs', item: {merger_id: merger.id, source: source}.to_s) if merger || source

      items = []
      if target_user.avatar_state == :none && from_user.avatar_state != :none
        [:avatar_image_source, :avatar_image_url, :avatar_image_updated_at, :avatar_state].each do |attr|
          items << merge_data.items.new(user: from_user, item_type: attr.to_s, item: from_user[attr]) if from_user[attr]
          target_user[attr] = from_user[attr]
        end
      end

      # record the users names and preferences in case of split.
      items << merge_data.items.new(user: from_user, item_type: 'user_name', item: from_user.name)
      items << merge_data.items.new(user: target_user, item_type: 'user_name', item: target_user.name)
      UserMergeDataItem.bulk_insert_objects(items)

      # bulk insert doesn't play nice with the hash values of preferences.
      merge_data.items.create!(user: from_user, item_type: 'user_preferences', item: from_user.preferences)
      merge_data.items.create!(user: target_user, item_type: 'user_preferences', item: target_user.preferences)

      if from_user.needs_preference_migration?
        prefs = shard_aware_preferences
      else
        copy_migrated_preferences # uses new rows to store preferences
        prefs = from_user.preferences
      end
      target_user.preferences = target_user.preferences.merge(prefs)
      target_user.save if target_user.changed?

      {'access_token_ids': from_user.access_tokens.shard(from_user).pluck(:id),
       'conversation_messages_ids': ConversationMessage.where(author_id: from_user, conversation_id: nil).shard(from_user).pluck(:id),
       'conversation_ids': from_user.all_conversations.shard(from_user).pluck(:id),
       'ignore_ids': from_user.ignores.shard(from_user).pluck(:id),
       'user_past_lti_id_ids': from_user.past_lti_ids.shard(from_user).pluck(:id),
       'Polling::Poll_ids': from_user.polls.shard(from_user).pluck(:id)}.each do |k, ids|
        merge_data.items.create!(user: from_user, item_type: k, item: ids) unless ids.empty?
      end
    end

    [:strong, :weak, :shadow].each do |strength|
      from_user.associated_shards(strength).each do |shard|
        target_user.associate_with_shard(shard, strength)
      end
    end

    copy_favorites
    populate_past_lti_ids
    handle_communication_channels
    destroy_conflicting_module_progressions
    move_enrollments
    move_observees

    Shard.with_each_shard(from_user.associated_shards + from_user.associated_shards(:weak) + from_user.associated_shards(:shadow)) do
      max_position = Pseudonym.where(user_id: target_user).order(:position).last.try(:position) || 0
      pseudonyms_to_move = Pseudonym.where(user_id: from_user)
      merge_data.add_more_data(pseudonyms_to_move)
      pseudonyms_to_move.update_all(["user_id=?, position=position+?", target_user, max_position])

      target_user.communication_channels.email.unretired.each do |cc|
        Rails.cache.delete([cc.path, 'invited_enrollments2'].cache_key)
      end

      handle_submissions

      from_user.all_conversations.find_each { |c| c.move_to_user(target_user) }

      # all topics changing ownership or with entries changing ownership need to be
      # flagged as updated so the materialized views update
      begin
        entries = DiscussionEntry.where(user_id: from_user)
        DiscussionTopic.where(id: entries.select(['discussion_topic_id'])).touch_all
        entries.update_all(user_id: target_user.id)
        DiscussionTopic.where(user_id: from_user).update_all(user_id: target_user.id, updated_at: Time.now.utc)
      rescue => e
        Rails.logger.error "migrating discussions failed: #{e}"
      end

      account_users = AccountUser.where(user_id: from_user)
      merge_data.add_more_data(account_users)
      account_users.update_all(user_id: target_user.id)

      attachments = Attachment.where(user_id: from_user)
      merge_data.add_more_data(attachments)
      Attachment.delay.migrate_attachments(from_user, target_user)

      updates = {}
      %w(access_tokens asset_user_accesses calendar_events collaborations
         context_module_progressions group_memberships ignores
         page_comments Polling::Poll rubric_assessments user_services
         web_conference_participants web_conferences wiki_pages).each do |key|
        updates[key] = "user_id"
      end
      updates['submission_comments'] = 'author_id'
      updates['conversation_messages'] = 'author_id'
      updates = updates.to_a
      version_updates = ['rubric_assessments', 'wiki_pages']
      updates.each do |table, column|
        begin
          klass = table.classify.constantize
          if klass.new.respond_to?("#{column}=".to_sym)
            scope = klass.where(column => from_user)
            klass.transaction do
              if version_updates.include?(table)
                update_versions(scope, table, column)
              end
              scope.update_all(column => target_user.id)
            end
          end
        rescue => e
          Rails.logger.error "migrating #{table} column #{column} failed: #{e}"
        end
      end

      context_updates = ['calendar_events']
      context_updates.each do |table|
        klass = table.classify.constantize
        klass.where(context_id: from_user, context_type: 'User').
          update_all(context_id: target_user.id, context_code: target_user.asset_string)
      end

      merge_data.bulk_insert_merge_data(data) unless data.empty?
      @data = []
      Enrollment.delay.recompute_due_dates_and_scores(target_user.id)
      target_user.update_account_associations
    end

    from_user.reload
    target_user.clear_caches
    from_user.destroy
    @merge_data.workflow_state = 'active'
    @merge_data.save!
  rescue => e
    @merge_data&.update_attribute(:workflow_state, 'failed')
    @merge_data.items.create!(user: target_user, item_type: 'merge_error', item: e.backtrace.unshift(e.message)) if @merge_data
    Canvas::Errors.capture(e, type: :user_merge, merge_data_id: @merge_data&.id, from_user_id: from_user&.id, target_user_id: target_user&.id)
    raise
  end

  def copy_favorites
    from_user.favorites.preload(:context).find_each do |f|
      target_user.favorites.find_or_create_by!(context: f.context, root_account_id: f.context.root_account_id)
    end
  end

  def translate_course_id_or_asset_string(key)
    id = key.is_a?(String) ? key.split('_').last : key
    new_id = Shard.relative_id_for(id, from_user.shard, target_user.shard)
    key.is_a?(String) ? [key.split('_').first, new_id].join('_') : new_id
  end

  # can remove when all preferences have been migrated
  def shard_aware_preferences
    return from_user.preferences if from_user.shard == target_user.shard
    preferences = from_user.preferences.dup
    %i{custom_colors course_nicknames}.each do |pref|
      preferences.delete(pref)
      new_pref = {}
      from_user.preferences.dig(pref)&.each do |key, value|
        new_key = translate_course_id_or_asset_string(key)
        new_pref[new_key] = value
      end
      preferences[pref] = new_pref unless new_pref.empty?
    end
    preferences
  end

  def copy_migrated_preferences
    target_user.migrate_preferences_if_needed # may as well
    from_values = from_user.user_preference_values.to_a
    target_values = target_user.user_preference_values.to_a.index_by{|r| [r.key, r.sub_key]}

    new_record_hashes = []
    existing_record_updates = {}

    from_values.each do |from_record|
      key = from_record.key
      sub_key = from_record.sub_key
      value = from_record.value
      if from_user.shard != target_user.shard
        # tl;dr do the same thing as shard_aware_preferences
        if key == "custom_colors"
          value = Hash[value.map{|id, color| [translate_course_id_or_asset_string(id), color]}]
        elsif key == "course_nicknames"
          sub_key = translate_course_id_or_asset_string(sub_key)
        end
      end

      if existing_record = target_values[[key, sub_key]]
        existing_record_updates[existing_record] = value
      else
        new_record_hashes << {:user_id => target_user.id, :key => key, :sub_key => sub_key&.to_json, :value => value.to_yaml}
      end
    end
    target_user.shard.activate do
      UserPreferenceValue.bulk_insert(new_record_hashes)
      existing_record_updates.each do |record, new_value|
        UserPreferenceValue.where(:id => record).update_all(:value => new_value)
      end
    end
  end

  def populate_past_lti_ids
    move_existing_past_lti_ids
    Shard.with_each_shard(from_user.associated_shards + from_user.associated_shards(:weak) + from_user.associated_shards(:shadow)) do
      lti_ids = []
      { enrollments: :course, group_memberships: :group, account_users: :account }.each do |klass, type|
        klass.to_s.classify.constantize.where(user_id: from_user).distinct_on(type.to_s + '_id').each do |context|
          next if UserPastLtiId.where(user: [target_user, from_user], context_id: context.send(type.to_s + '_id'), context_type: type.to_s.classify).exists?
          lti_ids << UserPastLtiId.new(user: target_user,
                                       context_id: context.send(type.to_s + '_id'),
                                       context_type: type.to_s.classify,
                                       user_uuid: from_user.uuid,
                                       user_lti_id: from_user.lti_id,
                                       user_lti_context_id: from_user.lti_context_id)
        end
      end
      UserPastLtiId.bulk_insert_objects(lti_ids)
    end
  end

  def move_existing_past_lti_ids
    existing_past_ids = target_user.past_lti_ids.select(:context_id, :context_type).shard(target_user).group_by(&:context_type)
    if existing_past_ids.present?
      ['Group', 'Account', 'Course'].each do |klass|
        next unless existing_past_ids[klass]
        Shard.partition_by_shard(existing_past_ids[klass]) do |shard_past_ids|
          UserPastLtiId.where(user_id: from_user, context_type: klass).where.not(context_id: shard_past_ids.map(&:context_id)).update_all(user_id: target_user.id)
        end
      end
    else
      # there are no possible conflicts just move them over
      Shard.partition_by_shard(from_user.past_lti_ids.shard(from_user).pluck(:id)) do |shard_past_ids|
        UserPastLtiId.where(id: shard_past_ids).update_all(user_id: target_user.id)
      end
    end
  end

  def handle_communication_channels
    max_position = target_user.communication_channels.last&.position&.+(1) || 0
    to_retire_ids = []
    known_ccs = target_user.communication_channels.pluck(:id)
    from_user.communication_channels.each do |cc|
      # have to find conflicting CCs, and make sure we don't have conflicts
      target_cc = detect_conflicting_cc(cc)

      if !target_cc && from_user.shard != target_user.shard
        User.clone_communication_channel(cc, target_user, max_position)
        new_cc = target_user.communication_channels.where.not(id: known_ccs).take
        known_ccs << new_cc.id
        merge_data.build_more_data([new_cc], user: target_user, workflow_state: 'non_existent', data: data)
      end

      next unless target_cc
      to_retire = identify_to_retire(cc, target_cc, max_position)
      if to_retire
        keeper = ([target_cc, cc] - [to_retire]).first
        copy_notificaion_policies(to_retire, keeper)
        to_retire_ids << to_retire.id
      end
    end

    finish_ccs(max_position, to_retire_ids)
  end

  def detect_conflicting_cc(source_cc)
    target_user.communication_channels.detect do |c|
      c.path.downcase == source_cc.path.downcase && c.path_type == source_cc.path_type
    end
  end

  def finish_ccs(max_position, to_retire_ids)
    if from_user.shard != target_user.shard
      handle_cross_shard_cc
    else
      from_user.shard.activate do
        ccs = CommunicationChannel.where(id: to_retire_ids).where.not(workflow_state: 'retired')
        merge_data.build_more_data(ccs, data: data) unless to_retire_ids.empty?
        ccs.update_all(workflow_state: 'retired') unless to_retire_ids.empty?
      end
      scope = from_user.communication_channels.where.not(workflow_state: 'retired')
      scope = scope.where.not(id: to_retire_ids) unless to_retire_ids.empty?
      unless scope.empty?
        merge_data.build_more_data(scope, data: data)
        scope.update_all(["user_id=?, position=position+?, root_account_ids='{?}'", target_user, max_position, target_user.root_account_ids])
      end
    end
    merge_data.bulk_insert_merge_data(data) unless data.empty?
    @data = []
  end

  def handle_cross_shard_cc
    ccs = from_user.communication_channels.where.not(workflow_state: 'retired')
    merge_data.build_more_data(ccs, data: data) unless ccs.empty?
    ccs.update_all(workflow_state: 'retired') unless ccs.empty?

    from_user.user_services.each do |us|
      new_us = us.clone
      new_us.shard = target_user.shard
      new_us.user = target_user
      new_us.save!
      merge_data.build_more_data([new_us], user: target_user, workflow_state: 'non_existent', data: data)
    end
    merge_data.build_more_data(from_user.user_services, data: data)
    from_user.user_services.delete_all
  end

  def identify_to_retire(source_cc, target_cc, max_position)
    # we prefer keeping the "most" active one, preferring the target user if they're equal
    # the comments inline show all the different cases, with the source cc on the left,
    # target cc on the right.  The * indicates the CC that will be retired in order
    # to resolve the conflict
    if target_cc.active?
      # retired, active
      # unconfirmed*, active
      # active*, active
      to_retire = source_cc
    elsif source_cc.active?
      # active, unconfirmed*
      # active, retired
      # target_cc will not be able to be restored on split, but it is either unconfirmed or retired so nbd
      target_cc.destroy_permanently!
      if from_user.shard != target_user.shard
        User.clone_communication_channel(source_cc, target_user, max_position)
      end
    elsif target_cc.unconfirmed?
      # unconfirmed*, unconfirmed
      # retired, unconfirmed
      to_retire = source_cc
    elsif source_cc.unconfirmed?
      # unconfirmed, retired
      # target_cc will not be able to be restored on split, but it is either unconfirmed or retired so nbd
      target_cc.destroy_permanently!
      if from_user.shard != target_user.shard
        User.clone_communication_channel(source_cc, target_user, max_position)
      end
    elsif
      # retired, retired
    to_retire = source_cc
    end
    to_retire
  end

  def copy_notificaion_policies(to_retire, keeper)
    # if the communication_channel is already retired, don't bother.
    return if to_retire.workflow_state == 'retired'
    time = Time.zone.now
    new_nps = []
    keeper.shard.activate do
      to_retire.notification_policies.where.not(notification_id: keeper.notification_policies.pluck(:notification_id)).each do |np|
        new_nps << NotificationPolicy.new(notification_id: np.notification_id,
                                          communication_channel_id: keeper.id,
                                          frequency: np.frequency,
                                          created_at: time,
                                          updated_at: time)
      end
      NotificationPolicy.bulk_insert_objects(new_nps)
    end
  end

  def move_observees
    merge_data.bulk_insert_merge_data(data) unless data.empty?
    @data = []
    observer_links = from_user.as_observer_observation_links.shard(from_user)
    # record all the records before destroying them
    # pass the from_user since user_id on observation_link is the student we are targeting the observer here
    merge_data.build_more_data(observer_links, user: from_user, data: data)

    Shard.partition_by_shard(observer_links) do |shard_observer_links|
      # delete links that are now duplicates between the from_user and target_user
      UserObservationLink.where(id: shard_observer_links).where(user_id: target_user.as_observer_observation_links.select(:user_id)).destroy_all
      # delete what would be observing themselves.
      UserObservationLink.where(id: shard_observer_links).where(user_id: target_user).destroy_all
      to_delete = target_user.as_student_observation_links.where(observer_id: from_user)
      merge_data.build_more_data(to_delete, data: data)
      to_delete.destroy_all
      # update links to target_user
      UserObservationLink.active.where(id: shard_observer_links).update_all(observer_id: target_user.id)
    end

    student_links = from_user.as_student_observation_links.shard(from_user)
    merge_data.build_more_data(student_links, data: data)
    Shard.partition_by_shard(student_links) do |shard_student_links|
      # delete links that are now duplicates between the from_user and target_user
      UserObservationLink.where(id: shard_student_links).where(observer_id: target_user.as_student_observation_links.select(:observer_id)).destroy_all
      # delete what would be observing themselves.
      UserObservationLink.where(id: shard_student_links).where(observer_id: target_user).destroy_all
      to_delete = target_user.as_observer_observation_links.where(user_id: from_user)
      merge_data.build_more_data(to_delete, data: data)
      to_delete.destroy_all
      # update links to target_user
      UserObservationLink.active.where(id: shard_student_links).update_all(user_id: target_user.id)
      target_user.as_student_observation_links.where(observer_id: shard_student_links).each(&:create_linked_enrollments)
    end

    # for any observers not already watching both users, make sure they have
    # any missing observer enrollments added
    if from_user.shard != target_user.shard
      from_user.shard.activate do
        UserObservationLink.where("user_id=?", target_user.id).where(id: data.map(&:context_id)).preload(:observer, :root_account).find_each do |link|
          # if the target_user is the same as the observer we already have a record
          next if Shard.shard_for(link.observer_id) == target_user.shard
          next if target_user.as_student_observation_links.active.where(observer_id: link.observer).for_root_accounts(link.root_account).exists?
          # create the record on the target users shard.
          new_link = UserObservationLink.create_or_restore(student: target_user, observer: link.observer, root_account: link.root_account)
          merge_data.build_more_data([new_link], user: target_user, workflow_state: 'non_existent', data: data)
        end
      end
    end

    merge_data.bulk_insert_merge_data(data) unless data.empty?
    @data = []
  end

  def destroy_conflicting_module_progressions
    # there is a unique index on the context_module_progressions table
    # we need to delete all the conflicting context_module_progressions
    # without impacting the users module progress and without having to
    # recalculate the progressions.
    # find all the modules progressions and delete the most restrictive
    # context_module_progressions
    ContextModuleProgression.
      where("context_module_progressions.user_id = ?", from_user.id).
      where("EXISTS (SELECT *
                     FROM #{ContextModuleProgression.quoted_table_name} cmp2
                     WHERE context_module_progressions.context_module_id=cmp2.context_module_id
                       AND cmp2.user_id = ?)", target_user.id).find_each do |cmp|

      ContextModuleProgression.
        where(context_module_id: cmp.context_module_id, user_id: [from_user, target_user]).
        order(Arel.sql("CASE WHEN workflow_state = 'completed' THEN 0
                       WHEN workflow_state = 'started' THEN 1
                       WHEN workflow_state = 'unlocked' THEN 2
                       WHEN workflow_state = 'locked' THEN 3
                       END DESC")).first.destroy
    end
  end

  def conflict_scope(column)
    other_column = (column == :user_id) ?  :associated_user_id : :user_id
    Enrollment.
      select("type, role_id, course_section_id, #{other_column}").
      group("type, role_id, course_section_id, #{other_column}").
      having("COUNT(*) > 1")
  end

  def enrollment_conflicts(enrollment, column, users)
    scope = Enrollment.
      where(type: enrollment.type,
            role_id: enrollment.role_id,
            course_section_id: enrollment.course_section_id)

    if column == :user_id
      scope = scope.where(user_id: users, associated_user_id: enrollment.associated_user_id)
    else
      scope = scope.where(user_id: enrollment.user_id, associated_user_id: users)
    end
    scope
  end

  def enrollment_keeper(scope)
    # prefer active enrollments to have no impact to the end user.
    # then just keep the newest one.
    scope.order(Arel.sql("CASE WHEN workflow_state='active' THEN 1
                          WHEN workflow_state='invited' THEN 2
                          WHEN workflow_state='creation_pending' THEN 3
                          WHEN workflow_state='completed' THEN 4
                          WHEN workflow_state='rejected' THEN 5
                          WHEN workflow_state='inactive' THEN 6
                          WHEN workflow_state='deleted' THEN 7
                          ELSE 8
                          END, updated_at DESC")).first
  end

  def update_enrollment_state(scope, keeper)
    # update the record on the target user to the better state of the from users enrollment
    enrollment_ids = Enrollment.where(id: scope).where.not(id: keeper).pluck(:id)
    Enrollment.where(:id => enrollment_ids).update_all(workflow_state: keeper.workflow_state)
    EnrollmentState.force_recalculation(enrollment_ids)

    # mark the would be keeper from the from_user as deleted so it will not be moved later
    keeper.destroy
  end

  def handle_conflicts(column)
    users = [from_user, target_user]

    # get each pair of conflicts and "handle them"
    conflict_scope(column).where(column => users).find_each do |e|

      # identify the other record that is conflicting with this one.
      scope = enrollment_conflicts(e, column, users)
      # get the highest state between the 2 users enrollments
      keeper = enrollment_keeper(scope)

      # identify if the target_users record needs promoted to better state
      to_update = scope.where.not(id: keeper).where(column => target_user)
      # if the target_users enrollment state will be updated pass the scope so
      # both target and from users records will be recorded in case of a split.
      if to_update.exists?
        # record both records state since both will change
        merge_data.build_more_data(scope, data: data)
        update_enrollment_state(scope, keeper)
      end

      # identify if the from users records are worse states than target user
      to_delete = scope.active.where.not(id: keeper).where(column => from_user)
      # record the current state in case of split
      merge_data.build_more_data(to_delete, data: data)
      # mark all conflicts on from_user as deleted so they will not be moved later
      to_delete.destroy_all
    end
  end

  def remove_self_observers
    # prevent observing self by marking them as deleted
    to_delete = Enrollment.active.where("type = 'ObserverEnrollment' AND
                                                   (associated_user_id = :target_user AND user_id = :from_user OR
                                                   associated_user_id = :from_user AND user_id = :target_user)",
                                                  {target_user: target_user, from_user: from_user})
    merge_data.build_more_data(to_delete, data: data)
    to_delete.destroy_all
  end

  def move_enrollments
    [:associated_user_id, :user_id].each do |column|
      Shard.with_each_shard(from_user.associated_shards) do
        Enrollment.transaction do
          handle_conflicts(column)
          remove_self_observers
          # move all the enrollments that have not been marked as deleted to the target user
          to_move = Enrollment.active.where(column => from_user)
          # upgrade to strong association if there are any enrollments
          target_user.associate_with_shard(from_user.shard) if to_move.exists?
          merge_data.build_more_data(to_move, data: data)
          to_move.update_all(column => target_user.id)
        end
      end
    end
    merge_data.bulk_insert_merge_data(data) unless data.empty?
    @data = []
  end

  def handle_submissions
    [
      [:assignment_id, :submissions],
      [:quiz_id, :'quizzes/quiz_submissions']
    ].each do |unique_id, table|
      begin
        # Submissions are a special case since there's a unique index
        # on the table, and if both the old user and the new user
        # have a submission for the same assignment there will be
        # a conflict.
        model = table.to_s.classify.constantize
        already_scope = model.where(:user_id => target_user)
        scope = model.where(:user_id => from_user)
        if model.name == "Submission"
          # we prefer submissions that have grades then submissions that have
          # a submission... that sort of makes sense.
          # we swap empty objects in cases of collision so that we don't
          # end up causing a unique index violation for a given assignment for
          # the either user, but also so we don't destroy submissions in case
          # of a user split.
          to_move_ids = scope.graded.select(unique_id).where.not(unique_id => already_scope.graded.select(unique_id)).pluck(:id)
          to_move_ids += scope.having_submission.
            select(unique_id).
            where.not(unique_id => already_scope.having_submission.select(unique_id)).
            where.not(id: to_move_ids).
            pluck(:id)
          to_move = scope.where(id: to_move_ids).to_a
          move_back = already_scope.where(unique_id => to_move.map(&unique_id)).to_a
          merge_data.build_more_data(to_move, data: data) unless to_move.empty?
          merge_data.build_more_data(move_back, data: data) unless move_back.empty?
          swap_submission(model, move_back, table, to_move, to_move_ids, 'fk_rails_8d85741475')
        elsif model.name == "Quizzes::QuizSubmission"
          subscope = already_scope.to_a
          to_move = model.where(user_id: from_user).joins(:submission).where(submissions: {user_id: target_user}).to_a
          move_back = model.where(user_id: target_user).joins(:submission).where(submissions: {user_id: from_user}).to_a

          to_move += scope.where("#{unique_id} NOT IN (?)", [subscope.map(&unique_id), move_back.map(&unique_id)].flatten).to_a
          move_back += already_scope.where(unique_id => to_move.map(&unique_id)).to_a
          merge_data.build_more_data(to_move, data: data)
          merge_data.build_more_data(move_back, data: data)
          swap_submission(model, move_back, table, to_move, to_move, 'fk_rails_04850db4b4')
        end
      rescue => e
        Rails.logger.error "migrating #{table} column user_id failed: #{e}"
      end
    end
    merge_data.bulk_insert_merge_data(data) unless data.empty?
    @data = []
  end

  def swap_submission(model, move_back, table, to_move, to_move_ids, fk)
    return if to_move_ids.empty?
    model.transaction do
      # there is a unique index on assignment_id and user_id. Unique
      # indexes are checked after every row during an update statement
      # to get around this and to allow us to swap we are setting the
      # user_id to the negative user_id and then the user_id, after the
      # conflicting rows have been updated.
      model.connection.execute("SET CONSTRAINTS #{model.connection.quote_table_name(fk)} DEFERRED")
      model.where(id: move_back).update_all(user_id: -from_user.id)
      model.where(id: to_move_ids).update_all(user_id: target_user.id)
      model.where(id: move_back).update_all(user_id: from_user.id)
      update_versions(model.where(id: to_move), table, :user_id)
      update_versions(model.where(id: move_back), table, :user_id)
    end
  end

  def update_versions(scope, table, column)
    scope.find_ids_in_batches do |ids|
      versionable_type = table.to_s.classify
      # TODO: This is a hack to support namespacing
      versionable_type = ['QuizSubmission', 'Quizzes::QuizSubmission'] if table.to_s == 'quizzes/quiz_submissions'
      version_ids = []
      Version.where(:versionable_type => versionable_type, :versionable_id => ids).find_in_batches(strategy: :cursor) do |versions|
        versions.each do |version|
          begin
            version_attrs = YAML.load(version.yaml)
            if version_attrs[column.to_s] == from_user.id
              version_attrs[column.to_s] = target_user.id
            end
            # i'm pretty sure simply_versioned just stores fields as strings, but
            # i haven't had time to verify that 100% yet, so better safe than sorry
            if version_attrs[column.to_sym] == from_user.id
              version_attrs[column.to_sym] = target_user.id
            end
            version.yaml = version_attrs.to_yaml
            version.save!
            if versionable_type == 'Submission'
              version_ids << version.id
            end
          rescue => e
            Rails.logger.error "migrating versions for #{table} column #{column} failed: #{e}"
            raise e unless Rails.env.production?
          end
        end
      end
      if version_ids.present?
        SubmissionVersion.where(version_id: version_ids, user_id: from_user).update_all(user_id: target_user.id)
      end
    end
  end
end