CREATE TABLE book (
    book_id     SERIAL PRIMARY KEY
    ---other stuff
);

CREATE TABLE loan (
    loan_id     SERIAL PRIMARY KEY,
    loan_date   DATE NOT NULL,
    return_date DATE,
    user_id     SERIAL REFERENCES library_user (user_id) ON UPDATE CASCADE ON DELETE RESTRICT
);

CREATE TABLE loan_book (
    loan_id     SERIAL REFERENCES loan (loan_id) ON UPDATE CASCADE,
    book_id     SERIAL REFERENCES book (book_id) ON UPDATE CASCADE,
    CONSTRAINT loan_book_id PRIMARY KEY (loan_id, book_id)
);

CREATE FUNCTION loan_count() RETURNS trigger AS $loan_count$
DECLARE
    max_book_count INTEGER := 3;
    book_count INTEGER := 0;
    fun_check BOOLEAN := false;

    BEGIN
        SELECT INTO book_count COUNT(*)
        FROM (
            SELECT * FROM loan NATURAL JOIN loan_book
            WHERE loan.loan_id = loan_book.loan_id
            AND loan.return_date > current_date) as subq
        WHERE subq.user_id = new.user_id;

        IF book_count >= max_book_count THEN
        RAISE EXCEPTION 'A user cannot borrow more than % books.', max_book_count;
        END IF;

       RETURN NEW;
    END;
$loan_count$ LANGUAGE plpgsql;

CREATE TRIGGER loan_count
BEFORE INSERT OR UPDATE ON loan
FOR EACH ROW EXECUTE PROCEDURE loan_count();