Select most recent results based on composite of fields

Programmer Question

I'm using MySQL, but I think this is a basic SQL question.

I don't know how else to ask but to give an example.

Say I have this data in my table:

id    date_time             foreign_key   key             value
1     2010-01-01 00:00:00   1             'temperature'   84
2     2010-01-01 00:00:01   1             'humidity'      34
3     2010-01-01 00:00:02   2             'temperature'   45
4     2010-01-01 00:00:03   2             'humidity'      23
5     2010-01-01 00:00:04   2             'dew_point'     78
6     2010-01-01 00:00:05   3             'temperature'   57
7     2010-01-01 00:00:06   3             'humidity'      41
8     2010-01-01 00:00:07   4             'temperature'   19
9     2010-01-01 00:00:08   4             'humidity'      35
10    2010-01-01 00:00:09   4             'dew_point'     24
11    2010-01-01 00:00:10   1             'temperature'   84
12    2010-01-01 00:00:11   1             'dew_point'     34
13    2010-01-01 00:00:12   2             'temperature'   45
14    2010-01-01 00:00:13   2             'humidity'      23
15    2010-01-01 00:00:14   3             'dew_point'     57
16    2010-01-01 00:00:15   3             'humidity'      41
17    2010-01-01 00:00:16   4             'temperature'   19
18    2010-01-01 00:00:17   4             'dew_point'     24

How do I get most recent of each key for a single foreign_key?

For Example, say I want the most recent for the foreign_key of 4,
the result I want would be:

id    date_time             foreign_key   key             value
9     2010-01-01 00:00:08   4             'humidity'      35
17    2010-01-01 00:00:16   4             'temperature'   19
18    2010-01-01 00:00:17   4             'dew_point'     24

What is the SQL I would use to achieve this result?

As an aside, I realize that this is not the first way most would choose to store data like this, but I have my reasons. Namely that this values are reported separately from each other.

Find the answer here