Troubleshooting a Rails API Database Connection Issue: A Personal Journey

The Challenge I Faced

I recently encountered a critical issue with my Rails backend that was reported as completely offline. The investigation report painted a grim picture: Rails API offline on port 3002, zero API routes registered, no database migrations found, and backend tests showing 0% coverage with 30 errors. What initially appeared to be a catastrophic system failure turned out to be a classic case of database connection misconfiguration. This is my journey through systematically diagnosing and resolving the issue.

Understanding the Architecture

My project structure consisted of a monorepo containing a Rails 8.0.2 backend configured to use PostgreSQL 15. The application was designed to run in a containerized environment with Docker Compose managing the various services. The Rails backend was supposed to serve API endpoints on port 3002, providing data for React frontend applications.

The Diagnostic Journey

Initial Discovery: Identifying the Database Mismatch

My first step was to examine the project structure to understand what I was working with:

ls -la
tree -I 'node_modules|tmp' --prune

This revealed a properly structured Rails application in the backend/ directory with all the expected components including models, controllers, and migration files. The presence of migration files contradicted the report claiming no migrations existed, which suggested the issue was about accessibility rather than absence.

Uncovering the PostgreSQL Configuration Mystery

I then checked the database configuration to understand what Rails expected:

cd backend && ls -la Gemfile* && head -20 Gemfile
cat config/database.yml | head -30

The configuration revealed that Rails was expecting PostgreSQL on localhost:5432 with default credentials (username: postgres, password: postgres). However, when I checked what was actually running:

docker ps -a | grep -E "postgres|pg"

I discovered a PostgreSQL container named pcvn-postgres running on port 5433 (not 5432) with completely different credentials. This mismatch was preventing Rails from establishing a database connection, which in turn prevented the entire API from starting.

Revealing the Actual Database Credentials

To find the correct credentials, I inspected the running container:

docker inspect pcvn-postgres | grep -A 20 '"Env"' | grep -E "POSTGRES_|PATH"

This revealed the actual credentials:

• Username: pcvn
• Password: pcvn_prod_db_2025_secure_password_8f3k9m2p
• Database: pcvn_erp
• Host port: 5433

Testing and Establishing the Connection

With the correct credentials identified, I tested the database connection:

DATABASE_HOST=localhost DATABASE_PORT=5433 \
DATABASE_USER=pcvn \
DATABASE_PASSWORD="pcvn_prod_db_2025_secure_password_8f3k9m2p" \
RAILS_ENV=development bin/rails db:create

The successful creation of backend_development database confirmed that Rails could now communicate with PostgreSQL.

Verifying Database Schema and Migrations

I then checked the migration status to understand the database schema state:

DATABASE_HOST=localhost DATABASE_PORT=5433 \
DATABASE_USER=pcvn \
DATABASE_PASSWORD="pcvn_prod_db_2025_secure_password_8f3k9m2p" \
RAILS_ENV=development bin/rails db:migrate:status

This revealed that all 11 migrations were already applied, showing the database schema was complete and ready. The supposed "missing migrations" were simply inaccessible due to the connection failure.

The Comprehensive Health Check

To ensure everything was working correctly before starting the server, I created a comprehensive health check script (test_rails.rb) that systematically verified each layer of the application stack. Running this script with the correct database credentials showed:

• Rails environment loaded successfully (version 8.0.2)
• Database connection established to backend_development
• All 15 database tables present with proper schema
• 94 routes defined, including 61 API endpoints
• Models properly loaded with associations configured
• Critical services like JwtService and ProductionMetricsService available
• ActionCable WebSocket support configured

Successfully Starting the Rails API

With all components verified as healthy, I started the Rails server:

DATABASE_HOST=localhost DATABASE_PORT=5433 \
DATABASE_USER=pcvn \
DATABASE_PASSWORD="pcvn_prod_db_2025_secure_password_8f3k9m2p" \
RAILS_ENV=development bin/rails server -p 3002

The server started successfully with Puma listening on port 3002. To verify the API was truly functional, I tested an authentication endpoint:

curl http://localhost:3002/api/v1/auth/me -I

The 401 Unauthorized response confirmed that the API was not only running but that its authentication middleware was functioning correctly, properly protecting secured endpoints.

Key Lessons Learned

This experience reinforced several important principles in my troubleshooting approach. First, I learned that reported errors often describe symptoms rather than root causes. The investigation report showed multiple catastrophic failures, but they all stemmed from a single database connection issue. Second, I discovered the importance of systematically verifying assumptions. The database, migrations, and routes all existed and were properly configured; they simply couldn't be accessed due to incorrect credentials.

The most valuable insight I gained was the importance of understanding containerized environments. In Docker-based development, the actual running configuration might differ from what's in configuration files, especially when multiple docker-compose files exist. By inspecting the running containers directly, I was able to find the ground truth about what credentials were actually in use.

The Resolution Impact

What initially appeared as a completely broken backend with multiple critical failures turned out to be a simple configuration mismatch. Once I provided Rails with the correct database connection parameters, everything else fell into place. The API routes registered properly, the models loaded correctly, and the entire backend became fully operational. This experience taught me that in complex systems, a single misconfiguration can cascade into what appears to be total system failure, and methodical investigation starting from the most fundamental components is often the fastest path to resolution.

Technical Takeaways

Working through this issue deepened my understanding of how Rails applications initialize and the critical role of database connectivity in that process. Rails attempts to establish database connections immediately upon startup, and without this connection, it cannot load models, register routes, or initialize services. This dependency chain explains why a simple port and credential mismatch manifested as such widespread failure.

The experience also highlighted the value of creating diagnostic tools like my health check script. Having a systematic way to verify each layer of the application stack made it much easier to identify what was actually broken versus what was simply inaccessible. This script has now become a permanent part of my debugging toolkit for Rails applications.

Through this troubleshooting journey, I transformed what seemed like a catastrophic backend failure into a fully functional API server, demonstrating that patient, systematic investigation paired with understanding of the underlying architecture can resolve even the most daunting-seeming issues.

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