ActiveRecord::ConnectionNotEstablished

ActiveRecord::ConnectionNotEstablished is raised when Rails cannot connect to the database. This can happen at startup or during runtime if the connection is lost.

Common causes

• Database server not running
• Incorrect database credentials
• Wrong database host/port
• Connection pool exhausted
• Network connectivity issues

How to fix it

• Verify database server is running
• Check database.yml credentials
• Test connection with rails db commands
• Increase connection pool size
• Check firewall/network settings

Example

# Error when database unavailable
User.first  # => ActiveRecord::ConnectionNotEstablished

# Check database.yml
default: &default
  adapter: postgresql
  host: <%= ENV.fetch("DATABASE_HOST", "localhost") %>
  username: <%= ENV.fetch("DATABASE_USER", "postgres") %>
  password: <%= ENV.fetch("DATABASE_PASSWORD", "") %>
  pool: <%= ENV.fetch("RAILS_MAX_THREADS", 5) %>

# Test connection
# $ rails db:ping  (Rails 7.1+)
# $ rails runner "ActiveRecord::Base.connection"

# Reconnect if connection lost
ActiveRecord::Base.connection_pool.with_connection do |conn|
  conn.execute("SELECT 1")
end

Related errors