How to add index to a large working app on Rails?

Well known fact is that PostgreSQL and many other RDBMS lock write access on the table while the index is being created. It is not acceptable when your project is large enough to allow a downtime for such the small adjustment like a new index.
There is a way to avoid the write-lock though. You can create the index concurrently. But how to do it with Rails Migrations?
First of all you can pass :algorithm option:

class AddIndexOnBatchIdToFundTrades < ActiveRecord::Migration[5.0]
  def change
    add_index :fund_trades, :batch_id, algorithm: :concurrently
  end
end

But when you try to run such the migration, you would get the following error:

PG::ActiveSqlTransaction: ERROR:  CREATE INDEX CONCURRENTLY cannot run inside a transaction block
: CREATE  INDEX CONCURRENTLY "index_fund_trades_on_batch_id" ON "fund_trades"  ("batch_id")

That's because any migration by default is executed inside a transaction. Thankfully there is a way to pass it through - use disable_ddl_transaction! to run your migration without a transaction:

class AddIndexOnBatchIdToFundTrades < ActiveRecord::Migration[5.0]
  disable_ddl_transaction!

  def change
    add_index :fund_trades, :batch_id, algorithm: :concurrently
  end
end