Fix ArgumentError: wrong number of arguments (given 2, expected 1) in Rails

This error occurs when a Ruby method receives more arguments than its signature allows. In Rails, this commonly happens when calling a model method, callback, or service object with extra parameters it was not designed to accept. Fix it by updating the method signature to accept the correct number of arguments or correcting the call site.

Reading the Stack Trace

Here's what each line means:

• app/services/notification_service.rb:8:in `send_notification': The send_notification method at line 8 expects 1 argument but received 2 from the caller.
• app/controllers/orders_controller.rb:24:in `create': The orders controller create action at line 24 calls send_notification with two arguments.
• wrong number of arguments (given 2, expected 1): Ruby's method dispatch detected a mismatch between the call site (2 args) and the method definition (1 arg).

Common Causes

class NotificationService
  def send_notification(user)
    Mailer.notify(user).deliver_later
  end
end

# Controller:
NotificationService.new.send_notification(user, order)  # 2 args, method takes 1

class Order < ApplicationRecord
  before_save :set_total

  private

  def set_total(order)  # Wrong: before_save callbacks receive no arguments
    self.total = items.sum(&:price)
  end
end

The Fix

Update the method signature to accept both the user and order arguments. This aligns the method definition with the call site so Ruby can dispatch the call correctly.

class NotificationService
  def send_notification(user)
    Mailer.notify(user).deliver_later
  end
end

# Controller:
NotificationService.new.send_notification(user, order)

class NotificationService
  def send_notification(user, order)
    Mailer.notify(user, order).deliver_later
  end
end

# Controller:
NotificationService.new.send_notification(user, order)

Testing the Fix

require 'rails_helper'

RSpec.describe NotificationService do
  describe '#send_notification' do
    let(:user) { User.create!(name: 'Alice', email: 'alice@example.com') }
    let(:order) { Order.create!(user: user, total: 99.99) }

    it 'accepts user and order arguments without raising' do
      service = NotificationService.new
      expect { service.send_notification(user, order) }.not_to raise_error
    end

    it 'enqueues a notification email' do
      service = NotificationService.new
      expect {
        service.send_notification(user, order)
      }.to have_enqueued_mail(Mailer, :notify)
    end
  end
end

Run your tests:

rspec

Pushing Through CI/CD

git checkout -b fix/argument-error-notification,git add app/services/notification_service.rb spec/services/notification_service_spec.rb,git commit -m "fix: update send_notification to accept user and order arguments",git push origin fix/argument-error-notification

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    services:
      postgres:
        image: postgres:15
        env:
          POSTGRES_DB: test_db
          POSTGRES_USER: postgres
          POSTGRES_PASSWORD: postgres
        ports:
          - 5432:5432
        options: >-
          --health-cmd pg_isready
          --health-interval 10s
          --health-timeout 5s
          --health-retries 5
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-ruby@v1
        with:
          ruby-version: '3.2'
          bundler-cache: true
      - run: bundle exec rails db:create db:migrate
        env:
          DATABASE_URL: postgres://postgres:postgres@localhost:5432/test_db
          RAILS_ENV: test
      - run: bundle exec rspec
        env:
          DATABASE_URL: postgres://postgres:postgres@localhost:5432/test_db
          RAILS_ENV: test

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

gem install bugstack

require 'bugstack'

Bugstack.init(api_key: ENV['BUGSTACK_API_KEY'])

Deploying the Fix (Manual Path)

1. Run rspec locally to confirm the method accepts the correct arguments.
2. Open a pull request with the updated method signature.
3. Wait for CI checks to pass on the PR.
4. Have a teammate review and approve the PR.
5. Merge to main and verify notifications work in staging.

Frequently Asked Questions