add unique select

[Jaymon]

Foo.unique("bar_id").gt__created(now - ago).lt__created(now).values()

Would result in:

SELECT DISTINCT bar_id WHERE _created > ... AND _created < ...

[Jaymon]

I added this one other time and then ended up removing it and I couldn't remember why because it was a few years ago but now I remember:

for SELECT DISTINCT, ORDER BY expressions must appear in select list

So if you want to say, get all the distinct foos that were active between January and March, you can't do a query like:

Activity.query.unique_foo().gt__created(january).lt__created(march).values()

Because it needs _created to be in the SELECT portion of the query (and the memories come flooding back), and if you add _created to the fields list like this: DISTINCT foo, _created it returns unique foo, _created pairs, which is almost never want I want.

[Jaymon]

This has been added but I'm not happy with the implementation, at this point it is buried in the Fields.options instance and so it's not very discoverable for later interfaces. The question is should this be made a first class method like count(), get(), and get_one()?

Basically I'm going to leave this open for a bit while I decide what is the best long term implementation.

[Jaymon]

I added Query.distinct() that you could do to mark all the selected fields as distinct/unique