In 1efd8828, ConnectionAdapters was put under...

In 1efd8828, ConnectionAdapters was put under eager_autoload. Due to the requires in that file, this caused ConnectionSpecification to be loaded, which references ActiveRecord::Base, which means the database connection is established. We do not want to connect to the database when Active Record is loaded, only when ActiveRecord::Base is first referenced by the user.

activerecord/lib/active_record.rb

@@ -42,7 +42,7 @@ module ActiveRecord
     autoload :ActiveRecordError, 'active_record/errors'
     autoload :ConnectionNotEstablished, 'active_record/errors'
     autoload :ConnectionAdapters, 'active_record/connection_adapters/abstract_adapter'
-    
+
     autoload :Aggregations
     autoload :Associations
     autoload :AttributeMethods
@@ -72,6 +72,7 @@ module ActiveRecord
     autoload :Persistence
     autoload :QueryCache
     autoload :Reflection
+    autoload :Result
     autoload :Schema
     autoload :SchemaDumper
     autoload :Serialization

activerecord/lib/active_record/base.rb

@@ -178,7 +178,7 @@ module ActiveRecord #:nodoc:
   # <tt>Person.find_all_by_last_name(last_name)</tt>.
   #
   # It's possible to add an exclamation point (!) on the end of the dynamic finders to get them to raise an
-  # <tt>ActiveRecord::RecordNotFound</tt> error if they do not return any records, 
+  # <tt>ActiveRecord::RecordNotFound</tt> error if they do not return any records,
   # like <tt>Person.find_by_last_name!</tt>.
   #
   # It's also possible to use multiple attributes in the same find by separating them with "_and_".
@@ -2163,4 +2163,5 @@ def clear_timestamp_attributes
   end
 end
 
+require 'active_record/connection_adapters/abstract/connection_specification'
 ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -4,20 +4,30 @@
 require 'active_support/core_ext/benchmark'
 require 'active_support/deprecation'
 
-# TODO: Autoload these files
-require 'active_record/connection_adapters/column'
-require 'active_record/connection_adapters/abstract/schema_definitions'
-require 'active_record/connection_adapters/abstract/schema_statements'
-require 'active_record/connection_adapters/abstract/database_statements'
-require 'active_record/connection_adapters/abstract/quoting'
-require 'active_record/connection_adapters/abstract/connection_pool'
-require 'active_record/connection_adapters/abstract/connection_specification'
-require 'active_record/connection_adapters/abstract/query_cache'
-require 'active_record/connection_adapters/abstract/database_limits'
-require 'active_record/result'
-
 module ActiveRecord
   module ConnectionAdapters # :nodoc:
+    extend ActiveSupport::Autoload
+
+    autoload :Column
+
+    autoload_under 'abstract' do
+      autoload :IndexDefinition,  'active_record/connection_adapters/abstract/schema_definitions'
+      autoload :ColumnDefinition, 'active_record/connection_adapters/abstract/schema_definitions'
+      autoload :TableDefinition,  'active_record/connection_adapters/abstract/schema_definitions'
+
+      autoload :SchemaStatements
+      autoload :DatabaseStatements
+      autoload :DatabaseLimits
+      autoload :Quoting
+
+      autoload :ConnectionPool
+      autoload :ConnectionHandler,       'active_record/connection_adapters/abstract/connection_pool'
+      autoload :ConnectionManagement,    'active_record/connection_adapters/abstract/connection_pool'
+      autoload :ConnectionSpecification
+
+      autoload :QueryCache
+    end
+
     # Active Record supports multiple database systems. AbstractAdapter and
     # related classes form the abstraction layer which makes this possible.
     # An AbstractAdapter represents a connection to a database, and provides an