一般的なRailsアプリケーションはconfig/database.ymlにデータベースへの接続情報があります。
これをつかってすでに存在しているデータベースからカラム情報を取得してみましょう。
必要なライブラリを読み込んでirbを起動します。
~ irb -r 'active_record' -r 'yaml'
データベースに接続します。
irb(main):001:0> ActiveRecord::Base.establish_connection YAML.load_file('config/database.yml')['development'] => #<ActiveRecord::ConnectionAdapters::ConnectionPool:0x00007fa3830aeb68 @automatic_reconnect=true, @available= #<ActiveRecord::ConnectionAdapters::ConnectionPool::ConnectionLeasingQueue:0x00007fa3830acd18 @cond=#<MonitorMixin::ConditionVariable:0x00007fa3830acb10 @cond=#<Thread::ConditionVariable:0x00007fa3830ac9d0>, @monitor=#<Monitor:0x00007fa3830adb00>>, @lock=#<ActiveRecord::ConnectionAdapters::ConnectionPool:0x00007fa3830aeb68 ...>, @num_waiting=0, @queue=[]>, @checkout_timeout=5.0, @connection_klass=ActiveRecord::Base, @connections=[], @db_config= #<ActiveRecord::DatabaseConfigurations::HashConfig:0x00007fa3a3444fc8 @configuration_hash={:adapter=>"sqlite3", :pool=>"<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>", :timeout=>5000, :database=>"db/development.sqlite3"}, @env_name="default_env", @name="primary">, @idle_timeout=300.0, @lock_thread=false, @mon_data=#<Monitor:0x00007fa3830adb00>, @mon_data_owner_object_id=380, @now_connecting=0, @pool_config= #<ActiveRecord::ConnectionAdapters::PoolConfig:0x00007fa3830bf5d0 @_mutex=#<Thread::Mutex:0x00007fa3830bf490>, @connection_klass=ActiveRecord::Base, @db_config= #<ActiveRecord::DatabaseConfigurations::HashConfig:0x00007fa3a3444fc8 @configuration_hash={:adapter=>"sqlite3", :pool=>"<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>", :timeout=>5000, :database=>"db/development.sqlite3"}, @env_name="default_env", @name="primary">, @pool=#<ActiveRecord::ConnectionAdapters::ConnectionPool:0x00007fa3830aeb68 ...>>, @query_cache_enabled= #<Concurrent::Map:0x00007fa3830ad9e8 entries=0 default_proc=#<Proc:0x00007fa3830ad0b0 /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/query_cache.rb:32>>, @reaper=#<ActiveRecord::ConnectionAdapters::ConnectionPool::Reaper:0x00007fa3830ac728 @frequency=60.0, @pool=#<ActiveRecord::ConnectionAdapters::ConnectionPool:0x00007fa3830aeb68 ...>>, @size=0, @thread_cached_conns=#<Concurrent::Map:0x00007fa3830aced0 entries=0 default_proc=nil>, @threads_blocking_new_connections=0> irb(main):002:0>
適当なクラスを定義します。
irb(main):002:0> class Computer < ActiveRecord::Base; end
カラム情報を取得しようとするのデータベースへの接続エラーが起きます。
irb(main):003:0> Computer.columns /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:210:in `block in wait_poll': could not obtain a connection from the pool within 5.000 seconds (waited 5.001 seconds); all pooled connections were in use (ActiveRecord::ConnectionTimeoutError) from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:199:in `loop' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:199:in `wait_poll' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:160:in `internal_poll' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:286:in `internal_poll' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:155:in `block in poll' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/3.0.0/monitor.rb:202:in `synchronize' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/3.0.0/monitor.rb:202:in `mon_synchronize' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:164:in `synchronize' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:155:in `poll' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:870:in `acquire_connection' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:588:in `checkout' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:428:in `connection' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:1128:in `retrieve_connection' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_handling.rb:327:in `retrieve_connection' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/connection_handling.rb:283:in `connection' from /Users/shigerunakajima/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activerecord-6.1.3.2/lib/active_record/model_schema.rb:553:in `load_schema!' ... 9 levels...
データベースへの接続情報が正しく取得できていないようです。
