Fix ActiveRecord::AssociationTypeMismatch: Comment(#123) expected, got String(#456) in Rails

This error occurs when you assign the wrong type to a polymorphic association or any typed association. Rails expects an instance of the associated model class but received a different type. Ensure you assign model instances, not raw strings or IDs, and that your polymorphic type and ID columns are set correctly together.

Reading the Stack Trace

Here's what each line means:

• activerecord (7.1.3) lib/active_record/associations/association.rb:285:in `raise_on_type_mismatch!': Rails checks the type of the assigned object and raises because it does not match the expected class.
• activerecord (7.1.3) lib/active_record/associations/belongs_to_association.rb:25:in `replace': The belongs_to writer method validates the type before setting the association.
• app/controllers/reactions_controller.rb:10:in `create': The controller assigns a string where a model instance is expected.

Common Causes

# Reaction belongs_to :reactable, polymorphic: true
reaction = Reaction.new
reaction.reactable = 'Comment'  # Should be a Comment instance, not a string

reaction = Reaction.new(
  reactable_type: 'Comments',  # Wrong: should be 'Comment' (singular)
  reactable_id: 1
)

def create
  @reaction = Reaction.new(reaction_params)
  @reaction.reactable = params[:reactable_type]  # Assigns a string
  @reaction.save!
end

The Fix

Use safe_constantize to convert the type string to a class and validate it against an allow list. Then find the actual record and assign the model instance to the polymorphic association instead of a raw string.

def create
  @reaction = Reaction.new(reaction_params)
  @reaction.reactable = params[:reactable_type]
  @reaction.save!
end

def create
  @reaction = Reaction.new(reaction_params)
  reactable_class = params[:reactable_type].safe_constantize
  if reactable_class && %w[Comment Post].include?(params[:reactable_type])
    @reaction.reactable = reactable_class.find(params[:reactable_id])
    @reaction.save!
    render json: @reaction, status: :created
  else
    render json: { error: 'Invalid reactable type' }, status: :unprocessable_entity
  end
end

Testing the Fix

require 'rails_helper'

RSpec.describe ReactionsController, type: :request do
  describe 'POST /reactions' do
    let(:comment) { create(:comment) }

    it 'creates a reaction for a valid reactable' do
      post '/reactions', params: {
        reactable_type: 'Comment',
        reactable_id: comment.id,
        emoji: 'thumbsup'
      }
      expect(response).to have_http_status(:created)
    end

    it 'rejects invalid reactable type' do
      post '/reactions', params: {
        reactable_type: 'InvalidModel',
        reactable_id: 1,
        emoji: 'thumbsup'
      }
      expect(response).to have_http_status(:unprocessable_entity)
    end
  end
end

Run your tests:

bundle exec rspec spec/requests/reactions_spec.rb

Pushing Through CI/CD

git checkout -b fix/rails-polymorphic-type-mismatch,git add app/controllers/reactions_controller.rb,git commit -m "fix: properly resolve polymorphic association from params",git push origin fix/rails-polymorphic-type-mismatch

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. Fix the polymorphic assignment to use model instances.
2. Add validation for allowed polymorphic types.
3. Write request specs for valid and invalid types.
4. Open a pull request.
5. Merge and verify in staging.

Frequently Asked Questions