Add new exception when attempting to connect to MySQL running on loca…

…lhost.

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -140,7 +140,11 @@ def text_type?(type)
         def connect
           @raw_connection = self.class.new_client(@connection_parameters)
         rescue ConnectionNotEstablished => ex
-          raise ex.set_pool(@pool)
+          if ex.message.match(/Can't connect to local server through socket/) && @connection_parameters[:host] == 'localhost'
+            raise CannotConnectThroughSocketOnLocalhost
+          else
+            raise ex.set_pool(@pool)
+          end
         end
 
         def reconnect

activerecord/lib/active_record/errors.rb

@@ -88,6 +88,16 @@ def set_pool(connection_pool)
   class ConnectionTimeoutError < ConnectionNotEstablished
   end
 
+  # Raised when a connection could not be obtained to the server running
+  # on localhost because it attempts to connect through a socket.
+  class CannotConnectThroughSocketOnLocalhost < ConnectionNotEstablished
+    def initialize(message = nil, connection_pool: nil)
+      message = 'The MySQL client library translates localhost address to a socket, so try to explicitly use the IP address (127.0.0.1) instead.'
+
+      super(message, connection_pool: connection_pool)
+    end
+  end
+
   # Raised when connection to the database could not been established because it was not
   # able to connect to the host or when the authorization failed.
   class DatabaseConnectionError < ConnectionNotEstablished