presence or coalesce_blank

[akimd]

I have had to write something like this:

Arel.when(at[:foo].blank).then(at[:bar]).else(at[:foo"])

except that foo and bar are much trickier to write. This makes the construct harder to read than needed. I would prefer to write something like

at[:foo].coalesce_blank(at[:bar])

or maybe

at[:foo].presence.coalesce(at[:bar])

where presence maps blanks to null.

Cheers!

[stackmystack]

Finally addressed in PR#90