Challenge 3: Reading a DATABASE_URL — Solution

# config/database.yml
production:
  adapter: postgresql
  url: <%= ENV["DATABASE_URL"] %>
  pool: <%= ENV.fetch("RAILS_MAX_THREADS", 5) %>

=begin
Notes:
- url: <%= ENV["DATABASE_URL"] %> reads the entire connection string (host,
  port, username, password, database name all encoded together) from one
  environment variable, rather than hardcoding any of those values into
  the checked-in database.yml file -- the hosting provider (Heroku,
  Render, etc.) sets this variable automatically when a database add-on is
  provisioned.
- ENV.fetch("RAILS_MAX_THREADS", 5) reads the pool size from its own
  environment variable if one is set, but falls back to a sensible default
  of 5 if it isn't -- fetch with a default argument, the same pattern
  Ruby's Hash#fetch already covered, applied here to ENV specifically.
- Sizing the connection pool to match the app server's thread count
  (rather than picking an arbitrary number) avoids two failure modes:
  threads sitting idle waiting for a connection that's smaller than the
  pool needs, or the database itself being overwhelmed by more open
  connections than it can handle.
=end