add preprocess option to find_in_batches in ActiveRecord adapter

[barelyknown]

The need to have a preprocessor was mentioned in this issue => #151

This pull request adds support for a :preprocess option for __find_in_batches that enables the user to specify a method to use to preprocess the batch.

The test isn't exactly great, but it does ensure that the preprocessor is getting called when it's defined in the options.

I can add documentation if you're good to merge this.

[karmi]

I think this makes sense! Can you rename the option to transport, since a similar option for import is called like that (see #113, source)?

[barelyknown]

Sure - I'd be happy to rename it. Given that transform operates at the instance level, maybe batch_transform is the right name for the method? That makes it most obvious in my opinion.

[karmi]

I think :transform would be fine... Can you add some realistic usage examples (preferably as docs, see eg. docs for import)? I would like to see an integration test for this as well, mainly as a way of documenting the feature.

[barelyknown]

Sure, I'll add documentation and an integration test. About the name, it can't be transform because that is already used. The options that get passed to import get passed through to __find_in_batches so the name can't be the same.

[karmi]

OK, got it. Then the preprocess name is of course much better.

---

BTW, one way how to work with the codebase in this case would be something like:

Article.__elasticsearch__.class_eval do
  def __find_in_batches(*args)
    [] # Do whatever here...
  end  
end 

All the other models would still have the default __find_in_batches implementation.

[barelyknown]

Documentation added plus a more descriptive test. Hopefully that looks good.

[karmi]

@barelyknown I'm sorry for letting this brew for such a long time... I'll try to have a look at it next week. Thanks!

[karmi]

@barelyknown I've retouched the code, tests and examples a bit in 351e3b7, thanks for the patch!, it's a useful feature.