Hi Gabriel. Thanks for that. It seemed a bit wrong to *not* be using combineValues, but a DoFn works just fine. 


On 5 April 2013 16:48, Gabriel Reid <gabriel.reid@gmail.com> wrote:
Hi Dave,

On Fri, Apr 5, 2013 at 5:05 PM, Dave Beech <dave@paraliatech.com> wrote:
I have a PGroupedTable<A,B> and I want to aggregate / combine the values to produce a PCollection<C> - in other words, I need the type of the aggregate to be different to the original value type. 

What's the best approach? The combineValues method takes either an Aggregator or a CombineFn but as far as I can see, both of these assume the end result will be of the same type as the values. 

The approach that I always use for this is just creating a custom DoFn to operate on the PGroupedTable and construct the instance of type C based in the incoming Iterable fromt he PGroupedTable. This basically works out to the same as a Aggregator.

I don't think that this scenario would be technically applicable to a CombineFn, because the CombineFn can be called any number of times on an incoming set of values, on both the map and reduce sides of a job. In order to map values to another type, the intermediate value of type C would somehow need to be given to the CombineFn each time it was used.

- Gabriel