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, 5 insertions, 0 deletions

diff --git a/activerecord/test/cases/query_cache_test.rb b/activerecord/test/cases/query_cache_test.rb
index 02ead8d914..0256a6da61 100644
--- a/activerecord/test/cases/query_cache_test.rb
+++ b/activerecord/test/cases/query_cache_test.rb
@@ -56,6 +56,11 @@ class QueryCacheTest < ActiveRecord::TestCase
   end
 
   def test_query_cache_is_applied_to_connections_in_all_handlers
+    ActiveRecord::Base.connection_handlers = {
+      writing: ActiveRecord::Base.default_connection_handler,
+      reading: ActiveRecord::ConnectionAdapters::ConnectionHandler.new
+    }
+
     ActiveRecord::Base.connected_to(role: :reading) do
       ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations["arunit"])
     end