Insert a group of consistent item with foreign key but colliding with existing items

Question

Is there a way to insert a group of items that are dependent but consistent between them with unique primary keys and foreign keys but that are colliding with items in database.

For example, given a table A:

id  primary key,
name

and a table B:

 id primary key,
 name,
 id_a'  -- foreign key on A

I want to insert:

INSERT INTO A(id, name) VALUES (1, "a");
INSERT INTO B(id, name, id_a) VALUES (1, "b", 1);

But the keys A.id or B.id could be taken already. Is there a way to insert my elements with auto setting key and foreign keys in a consistent way without colliding with existing elements?

Answer 1

In reference to your comment, CTEs are scalable that way. You should do everything in a single transaction anyway. The related question @ypercube suggested should fit your case exactly:
How do I insert a row which contains a foreign key?

Your other option would be to employ lastval() and curval() to identify values most recently obtained from sequences. Not providing code, your question is too vague. These related questions on SO may be helpful:
Reference value of serial column in another column during same INSERT
PostgreSQL next value of the sequences?