Passing a record as function argument PL/pgSQL

Question

First I am really new to pl/pgsql. Need it for a project.

I am stuck with this (simplified) problem.

My db schema has a n to m relationship (author, books, a

Answer 1

Passing JSON datatype (Postgresql 9.2 or higher):

CREATE OR REPLACE FUNCTION f_insert_book(_book json, _authors json)
   RETURNS void AS 
$$
BEGIN
-- insert book into table books
Insert into books values select * from json_populate_recordset(null:book, _book);
    -- for each author add an entry to author_books table
Insert into authors values select * from json_populate_recordset(null:authors, _authors);
end;
$$ language plpgsql;

Answer 2

Using unnest() and a data-modifying CTE (requires Postgres 9.1 or later), this can be a simple SQL query:

WITH x AS (SELECT '(1,foo_book)'::book AS _book
                , '{1,2,3}'::int[]     AS _authors)
   , y AS (
   INSERT INTO book  -- no column list, correct due to composite type
   SELECT (x._book).*
   FROM   x
   RETURNING book_id
   )
INSERT INTO author_book (book_id, author_id)
SELECT y.book_id, unnest(x._authors)
FROM   x,y;  -- CROSS JOIN ok, only 1 row for x and y

The first CTE x is just for simplified data input and not strictly needed.

SQL Fiddle.

As to your questions:

Question 1: Is the function outline "correct"/does it make sense?

Might be easier to pass base types instead of the composite type book, but it is a perfectly valid approach. You have to know your way around the syntax for complex types, though. For instance, note the parenthesis around the name in my example: (x._book).*.

A plpgsql function could look like this:

CREATE OR REPLACE FUNCTION f_insert_book(_book book, _authors integer[])
   RETURNS void AS 
$func$
BEGIN
    WITH y AS (
        INSERT INTO book b
        SELECT (_book).*
        RETURNING b.book_id
        )
    INSERT INTO author_book (book_id, author_id)
    SELECT y.book_id, unnest(_authors)
    FROM   y;
END
$func$ LANGUAGE plpgsql;

Question 2: How to insert record book into table book? (...) or is there a "smarter" way?

The smarter way is to decompose the composite type with (variable_name).*.

As the type is guaranteed to match the table (being derived from it), this is one of the rare cases, where it is perfectly ok, not to provide a column list for the INSERT command in persisted code.

Question 3: How would I call my function insert_book? ...

SELECT f_insert_book('(1,foo_book)'::book, '{1,2,3}'::int[]);

Within other plpgsql functions, use PERFORM instead of SELECT if you don't provide a target (INTO foo) for the (non-existing) results.