Heroku Push Rejection: Analysis and Resolution of pre-receive hook declined Error

Error Background and Phenomenon Analysis

When developers attempt to deploy Rails applications to the Heroku platform using the git push heroku master command, they frequently encounter push rejections with the error message: ! [remote rejected] master -> master (pre-receive hook declined). This error indicates that Heroku's pre-receive hook has detected an issue and rejected the code push.

Core Problem Diagnosis

Based on the key hint in the error log ! Heroku push rejected, no Cedar-supported app detected, it can be determined that Heroku fails to recognize the current codebase as a supported application type. Cedar is Heroku's runtime environment stack that requires specific project structures and configuration files for proper identification.

Project Structure Verification

First, confirm that the Rails application is located at the root of the Git repository. Use the ls -la command to inspect the current directory structure, ensuring visibility of standard Rails project files:

app/
config/
db/
Gemfile
Gemfile.lock
...

If the application resides in a subdirectory, adjust the project structure or use Heroku's buildpack configuration to specify the correct path.

Gemfile Configuration Check

Heroku relies on the Gemfile to identify Ruby applications. Ensure this file:

• Has the correct filename and is located in the project root
• Contains necessary gem dependency declarations
• Has correct syntax without parsing errors
• Maintains consistency with the Gemfile.lock file

Verify Gemfile configuration correctness by running bundle install.

Git Repository Status Verification

Use the git status command to check if all necessary files are properly committed to version control:

$ git status
On branch master
nothing to commit, working tree clean

Ensure all critical files (particularly Gemfile, Gemfile.lock, and application source code) are added to Git tracking and committed. If there are uncommitted changes, complete the commit using git add . and git commit -m "Deployment preparation".

Supplementary Solutions

When basic checks fail to resolve the issue, consider these advanced approaches:

Branch Push Testing

Create and push a temporary branch to isolate the problem:

git checkout -b temp_deploy_branch
git push heroku temp_deploy_branch:master

This method helps exclude issues caused by master branch-specific configurations.

Buildpack Configuration

If Heroku cannot automatically detect the application type, explicitly set the buildpack:

heroku buildpacks:set https://github.com/heroku/heroku-buildpack-ruby

For Rails 3.2 applications, ensure compatibility with Ruby versions and buildpack configurations.

Log Analysis

Use the heroku logs --tail command to view deployment logs in real-time, searching for specific error messages. Common errors include:

• CSS or SCSS syntax errors
• Database configuration issues
• Missing runtime dependencies

Web Interface Inspection

Log into the Heroku dashboard and examine detailed build logs under the application's Activity tab. The web interface typically provides more user-friendly error display and diagnostic information than the command line.

Preventive Measures and Best Practices

To avoid similar deployment issues, adhere to the following development standards:

• Use the same Ruby and Rails versions in local and production environments
• Regularly run bundle audit to check for security vulnerabilities
• Execute a complete test suite before deployment: rails test
• Use .gitignore to properly exclude development environment-specific files (e.g., .DS_Store)
• Maintain timely updates and compatibility checks for Gemfile dependencies

Conclusion

The Heroku pre-receive hook rejection error typically stems from project structure or configuration issues. Through a systematic diagnostic process—from basic project structure verification to advanced log analysis—developers can effectively locate and resolve deployment obstacles. The key lies in ensuring Heroku can correctly identify the application type and access all necessary deployment resources.