Scenario:

I wrote CSV data with something like

```
df.write.partitionBy("foo", "bar").csv("hdfs:///quux/bletch")

```

The CSV files in the `hdfs://quux/bletch/foo=baz/bar=moo` directories all lack the `foo` and `bar` columns.

How can I read that back in to get these columns back? Please don't tell me I have to read every directory individually, add the data manually and do a big, fat `union`...