Fix ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR: column posts.comments_count does not exist in Rails

This error occurs when you enable counter_cache on an association but the parent table does not have the required counter column. Rails expects a column named associations_count on the parent model table. Add the column with a migration and optionally reset counters for existing records using reset_counters.

Reading the Stack Trace

Here's what each line means:

• activerecord (7.1.3) lib/active_record/counter_cache.rb:38:in `update_counters': Rails tries to increment the counter cache column but it does not exist in the database.
• activerecord (7.1.3) lib/active_record/associations/belongs_to_association.rb:120:in `update_counters': The belongs_to association triggers the counter cache update on create.
• app/models/comment.rb:3:in `<class:Comment>': The Comment model declares counter_cache: true on its belongs_to :post association.

Common Causes

class Comment < ApplicationRecord
  belongs_to :post, counter_cache: true
  # But posts table does not have a comments_count column
end

# Migration added 'comment_count' but Rails expects 'comments_count'
class Comment < ApplicationRecord
  belongs_to :post, counter_cache: true  # Expects comments_count
end

# posts.comments_count is 0 but the post actually has 15 comments
# This happens after bulk imports or direct SQL updates

The Fix

Add the comments_count column to the posts table with a default of 0. After running the migration, reset counters for existing records so the cached counts match the actual number of comments.

class Comment < ApplicationRecord
  belongs_to :post, counter_cache: true
end
# Missing migration for comments_count column

# 1. Generate migration:
# rails generate migration AddCommentsCountToPosts comments_count:integer

class AddCommentsCountToPosts < ActiveRecord::Migration[7.1]
  def change
    add_column :posts, :comments_count, :integer, default: 0, null: false
  end
end

# 2. Reset counters for existing records:
# rails runner 'Post.find_each { |p| Post.reset_counters(p.id, :comments) }'

Testing the Fix

require 'rails_helper'

RSpec.describe Comment, type: :model do
  describe 'counter cache' do
    let(:post) { create(:post) }

    it 'increments comments_count when a comment is created' do
      expect { create(:comment, post: post) }.to change {
        post.reload.comments_count
      }.by(1)
    end

    it 'decrements comments_count when a comment is destroyed' do
      comment = create(:comment, post: post)
      expect { comment.destroy }.to change {
        post.reload.comments_count
      }.by(-1)
    end
  end
end

Run your tests:

bundle exec rspec spec/models/comment_spec.rb

Pushing Through CI/CD

git checkout -b fix/rails-counter-cache,git add db/migrate/ app/models/comment.rb,git commit -m "fix: add comments_count column for counter cache",git push origin fix/rails-counter-cache

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:16
        env:
          POSTGRES_PASSWORD: postgres
        ports: ['5432:5432']
    steps:
      - uses: actions/checkout@v4
      - uses: ruby/setup-ruby@v1
        with:
          ruby-version: '3.3'
          bundler-cache: true
      - run: bin/rails db:setup
      - run: bundle exec rspec

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. Create and run the migration to add the counter column.
2. Reset counters for all existing records.
3. Run model specs verifying counter increments and decrements.
4. Open a pull request.
5. Merge, run migration on production, and reset counters.

Frequently Asked Questions