a trigger question

Give two tables A and B. B has a field that references A's primary key.

For example:

create table A(
i int not null,
s text,
primary key(i));

create table B(
i int not null,
s text,
primary key(i),
foreign key(i) references A(i));

I like to create a trigger on table A. When a new row is inserted into A
(ex: with i = 5), I like to have the trigger inserts a new row in table B
whose field "i" has the same value as that of the A's (ex: i = 5).

As I do this, the error message is something like: "referential integration
violation - key referenced in B not found in A". This makes sense to me
since at the time the trigger inserts in B, A's new row is not visible yet
-- not committed yet.

How can I solve this problem using trigger?

Thanks!

Lixin Zhou