Create hub_connection collect method

[annakrystalli]

Thanks for the report @elray1 ! This is a result of a conscious prior decision to not error when connecting to an empty hub. Using connect_hub on an empty hub still loads config metadata but the subclass is an empty list instead of an arrow dataset. Hence dplyr::collect doesn't have a method for it and fails.

This would likely be non-ideal to fix from within connect_hub() but it made me think of a somewhat related discussion @nickreich had raised a while ago (transferred to an issue in this repo: #13 ) in which the ability to run as_model_out_tbl() on the results of collect by default was proposed through a collect_hub() function. Such a function would be an ideal place to handle the empty hub exception too.

Note that one thing we could also do is rather than have a different function (collect_hub()) we could have a collect() S3 method of our own that is dispatched on hub-connection class objects.

Originally posted by @annakrystalli in #4

[annakrystalli]

The method will:

• Check if the hub_connection object is an empty list and return NULL with a warning if so.
• Otherwise, call dplyr::collect() with next method.
• If dplyr::collect() succeeds, try as_model_out_tbl() tibble on output. If it succeeds return return model_out_tbl, otherwise return original tbl with silenceable message.

The function will resolve #4 and #13

[annakrystalli]

Sadly it is not possible to create a hub_connection specific method because query objects passed to collect do not retain the hub_connection class (the have an arrow_dplyr_query class). So the above approach would only work in the minority of cases when collect() is run directly on a hub_connection object to return all hub data.

As such I have gone ahead an implemented the initial idea of a collect_hub() wrapper around dplyr::collect() that also coerces to model_out_tbl where possible.