truncate command for dates

[mlucy]

@coffeemug asked for this and can maybe give more information.

[coffeemug]

Currently I can run:

> r.now().day()
2

> r.now().month()
2

// etc.

I want to be able to do:

> r.now().day({prefix: true})
"2015-02-02"

> r.now().month({prefix: true})
"2015-02"

// etc.

This would make it easier to group by month or day, whereas right now I would have to construct the group manually and it's enough of a pain to do it that RethinkDB isn't a great tool for such operations.

[coffeemug]

Also note, the above is meant to illustrate what I want to do, not to dictate the API. We might want to come up with a different API to do it (for example, do we want to return strings or something else?), but I don't understand dates and times well enough to propose something sensible.

[marshall007]

I'd much rather see a command that allows arbitrary formatting, something like:

> r.now().format("YYYY-MM-DD")
"2015-02-02"

> r.now().format("YYYY-MM")
"2015-02"

[VeXocide]

date has a nice format that might work, http://man7.org/linux/man-pages/man1/date.1.html.

[coffeemug]

I really like the format command.

One concern I have with it is that whenever I format things like this I always find myself digging through documentation to find a correct string to represent what I want, which always annoys me. It doesn't seem very... conducive to quick and dirty time series grouping queries, though it might be a matter of good docs.

[VeXocide]

For the record, this is probably related to #3353 and #3387.

[danielmewes]

I think I would prefer a format term over having a prefix opt arg.
Even though you might have to look up the placeholders to use in the format string, it is much more flexible and has a lot of other use cases as well.

[marshall007]

@coffeemug: One concern I have with it is that whenever I format things like this I always find myself digging through documentation to find a correct string to represent what I want, which always annoys me.

Agreed, I think one solution would be to allow passing a "human-readable" unit (i.e. year, month, day, etc) which would give you an ISO 8601 formatted date up to and including the unit specified.

So .format('day') would just be an alias to .format('YYYY-MM-DD').

[timmaxw]

I think that YYYY-MM-DD is an easy enough format string to remember. The problem that I usually have with date format strings is that they look like %Y-%m-%d, not to be confused with %y-%m-%d, %Y-%M-%d, or %Y-%m-%D, all of which are valid format strings that do different things. If we design our format syntax so that YYYY-MM-DD literally works and does what it looks like, I think it would be OK.