Raise helpful error when role doesn't exist

If you try to call `connected_to` with a role that doesn't have an
established connection you used to get an error that said:

```
>> ActiveRecord::Base.connected_to(role: :i_dont_exist) { Home.first }

ActiveRecord::ConnectionNotEstablished Exception: No connection pool
with 'primary' found.
```

This is confusing because the connection could be established but we
spelled the role wrong.

I've changed this to raise if the `role` used in `connected_to` doesn't
have an associated handler. Users who encounter this should either check
that the role is spelled correctly (writin -> writing), establish a
connection to that role in the model with connects_to, or use the
`database` keyword for the `role`.

I think this will provide a less confusing error message for those
starting out with multiple databases.

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
index 79c2c4d827..0b3fb82e12 100644
--- a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
@@ -328,6 +328,16 @@ module ActiveRecord
       ensure
         ActiveRecord::Base.connection_handlers = original_handlers
       end
+
+      def test_calling_connected_to_on_a_non_existent_handler_raises
+        error = assert_raises ArgumentError do
+          ActiveRecord::Base.connected_to(role: :reading) do
+            yield
+          end
+        end
+
+        assert_equal "The reading role does not exist. Add it by establishing a connection with `connects_to` or use an existing role (writing).", error.message
+      end
     end
   end
 end