incremental invoice numbers without gap in rails + postgresql

migration.rb

class CreateInvoiceNumbers < ActiveRecord::Migration
  def up
    create_table :invoice_numbers do |t|
      t.integer :year, null: false, unique: true
      t.integer :next_number_within_year, null: false, default: 1
    end

    add_index :invoice_numbers, :year, unique: true

    (2016..2045).each do |year|
      ::InvoiceNumber.create!(year: year)
    end

    create_table :payment_statements do |t|
      t.integer :user_id, null: false
      t.integer :payment_request_id, null: false
      t.integer :number, null: false
      t.integer :year, null: false
      t.timestamps
    end
    add_index :payment_statements, [:year, :number], unique: true
    add_index :payment_statements, :payment_request_id, unique: true

    execute %{
      CREATE OR REPLACE FUNCTION add_invoice_number()
        RETURNS trigger AS
      $BODY$
      DECLARE
        new_invoice_number INTEGER;
      BEGIN
        LOCK TABLE invoice_numbers IN ACCESS EXCLUSIVE MODE;

        SELECT invoice_numbers.next_number_within_year
          INTO new_invoice_number
          FROM invoice_numbers
          WHERE invoice_numbers.year = DATE_PART('YEAR', NEW.created_at)
        ;

        NEW.number = new_invoice_number;
        NEW.year   = DATE_PART('YEAR', NEW.created_at);
        
        UPDATE invoice_numbers
          SET next_number_within_year = new_invoice_number + 1
          WHERE year = DATE_PART('YEAR', NEW.created_at);

        RETURN NEW;
      END;
      $BODY$
      LANGUAGE plpgsql;
    }

    execute %{
      CREATE TRIGGER add_invoice_number_to_statements
      BEFORE INSERT
      ON payment_statements
      FOR EACH ROW
      EXECUTE PROCEDURE add_invoice_number();
    }
  end

  def down
    drop_table :invoice_numbers
    drop_table :payment_statements
    execute %{
      DROP FUNCTION add_invoice_number();
    }
  end
end