How to insert in multiple tables with a single API call in Supabase

Is there any reason you need to do this with a single call? I'm assuming from your structure that you're not going to create a new genre for every book you create, so most of the time, you're just inserting a book record and a book_gen_rel record. In the real world, you're probably going to have books that fall into multiple genres, so eventually you're going to be changing your function to handle the insert of a single book along with multiple genres in a single call.

That being said, there are two ways too approach this. You can make multiple API calls from the client (and there's really no problem doing this -- it's quite common). Second, you could do it all in a single call if you create a PostgreSQL function and call it with .rpc().

Example using just client calls to insert a record in each table:

    const { data: genre_data, error: genre_error } = await supabase
      .from('genre')
      .insert([
        { name: 'Technology' }
      ]);
    const genre_id = genre_data[0].id;  
    const { data: book_data, error: book_error } = await supabase
      .from('book')
      .insert([
        { name: 'The Joys of PostgreSQL' }
      ]);
    const book_id = book_data[0].id;
    const { data: book_genre_rel_data, error: book_genre_rel_error } = await supabase
      .from('book_genre_rel_data')
      .insert([
        { book_id, genre_id }
      ]);

Here's a single SQL statement to insert into the 3 tables at once:

WITH genre AS (
  insert into genre (name) values ('horror') returning id
),
book AS (
  insert into book (name) values ('my scary book') returning id
)
insert into book_genre_rel (genre_id, book_id) 
  select genre.id, book.id from genre, book

Now here's a PostgreSQL function to do everything in a single function call:

CREATE OR REPLACE FUNCTION public.insert_book_and_genre(book_name text, genre_name text)
    RETURNS void language SQL AS
$$
  WITH genre AS (
    insert into genre (name) values (genre_name) returning id
  ),
  book AS (
    insert into book (name) values (book_name) returning id
  )
  insert into book_genre_rel (genre_id, book_id) 
    select genre.id, book.id from genre, book
$$

Here's an example to test it:

select insert_book_and_genre('how to win friends by writing good sql', 'self-help')

Now, if you've created that function (inside the Supabase Query Editor), then you can call it from the client like this:

const { data, error } = await supabase
  .rpc('insert_book_and_genre', {book_name: 'how I became a millionaire at age 3', genre_name: 'lifestyle'})

Again, I don't recommend this approach, at least not for the genre part. You should insert your genres first (they probably won't change) and simplify this to just insert a book and a book_genre_rel record.