Can you get DB username, pw, database name in Rails?

From within rails you can create a configuration object and obtain the necessary information from it:

config   = Rails.configuration.database_configuration
host     = config[Rails.env]["host"]
database = config[Rails.env]["database"]
username = config[Rails.env]["username"]
password = config[Rails.env]["password"]

See the documentation for Rails::Configuration for details.

This just uses YAML::load to load the configuration from the database configuration file (database.yml) which you can use yourself to get the information from outside the rails environment:

require 'YAML'
info = YAML::load(IO.read("database.yml"))
print info["production"]["host"]
print info["production"]["database"]
...

Bryan's answer in the comment above deserves a little more exposure:

>> Rails.configuration.database_configuration[Rails.env]
=> {"encoding"=>"unicode", "username"=>"postgres", "adapter"=>"postgresql", "port"=>5432, "host"=>"localhost", "password"=>"postgres", "database"=>"mydb", "pool"=>5}

ActiveRecord::Base.connection_config

returns the connection configuration in a hash:

=> {:adapter=>ADAPTER_NAME, :host=>HOST, :port=>PORT, 
    :database=>DB, :pool=>POOL, :username=>USERNAME, 
    :password=>PASSWORD} 

As tpett remarked in their comment: this solution accounts for merging the configuration from database.yml and from the environment variable DATABASE_URL.

I think this is the simplest solution. After some testing (in Rails 5.2 at least) this will resolve DATABASE_URL correctly.

 ActiveRecord::Base.configurations[Rails.env]