array_to_json/array_agg question

Hi

Before I go down the road of taking care of this in the front-end through iterations, I thought I would ask the pgsql if there was a clever query I could do on postgres that would take care of it for me instead.

In essence, I would like to consolidate values from the same key as a json array, so instead of :
[{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]
I would have (forgive me if my JSON syntax is incorrect here) :
[{"key":"one",[{"value_1":"foo","value_2":"foo"},{"value_1":"foo","value_2":"bar"}]}]

A simplified example of where I am at the moment:

create table test_a(key text,value_1 text,value_2 text);insert into test_a(key,value_1,value_2) values('one','foo','foo');
insert into test_a(key,value_1,value_2) values('one','foo','bar');
insert into test_a(key,value_1,value_2) values('two','bar','foo');
select array_to_json(array_agg(row_to_json(p))) from (select * from test_a where key='one') p;
[{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]