Revert "Merge branch 'rs-remove-bad-foreground-migration' into 'master'"

This reverts merge request !14240

db/migrate/20170828135939_migrate_user_external_mail_data.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class MigrateUserExternalMailData < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class User < ActiveRecord::Base
+    self.table_name = 'users'
+
+    include EachBatch
+  end
+
+  class UserSyncedAttributesMetadata < ActiveRecord::Base
+    self.table_name = 'user_synced_attributes_metadata'
+
+    include EachBatch
+  end
+
+  def up
+    User.each_batch do |batch|
+      start_id, end_id = batch.pluck('MIN(id), MAX(id)').first
+
+      execute <<-EOF
+        INSERT INTO user_synced_attributes_metadata (user_id, provider, email_synced)
+        SELECT id, email_provider, external_email
+        FROM users
+        WHERE external_email = TRUE
+        AND NOT EXISTS (
+          SELECT true
+          FROM user_synced_attributes_metadata
+          WHERE user_id = users.id
+          AND provider = users.email_provider
+        )
+        AND id BETWEEN #{start_id} AND #{end_id}
+      EOF
+    end
+  end
+
+  def down
+    UserSyncedAttributesMetadata.each_batch do |batch|
+      start_id, end_id = batch.pluck('MIN(id), MAX(id)').first
+
+      execute <<-EOF
+        UPDATE users
+        SET users.email_provider = metadata.provider, users.external_email = metadata.email_synced
+        FROM user_synced_attributes_metadata as metadata, users
+        WHERE metadata.email_synced = TRUE
+        AND metadata.user_id = users.id
+        AND id BETWEEN #{start_id} AND #{end_id}
+      EOF
+    end
+  end
+end