Question

In a rails 3 app I’m building I have these three classes:

class Instrument < ActiveRecord::Base
  has_many :parts
  has_many :pieces, through: :parts
end    
class Part < ActiveRecord::Base
  belongs_to :instrument
  belongs_to :piece
end
class Piece < ActiveRecord::Base
  has_many :parts
  has_many :instruments, through: :parts
end

I am looking for a way to select all Pieces of which the associated instruments are a subset of an arbitrary set of instruments.

To avoid confusion I’ll give an example: given

• Piece(id:1) with instruments: [1,2,3],
• Piece(id:2) with [1,2,3,4]
• Piece(id:3) with [1,3,4]
• Piece(id:4) with [2],

somequery(1,2,3,4) should yield all four pieces, somequery(1,2,3) only 1 and 4, somequery(1,3,4) only piece 3 and somequery(2) only 4.

I’m using rails 3.2 and a solution in rails would be great but sql is also fine, and preferably postgres, but if there is a mysql-specific solution to this that would also be fine. Furthermore I’m not working with a relatively small database (1000+ pieces, 50 instruments and 15.000 parts) which isn’t heavily queried, so if the query is not optimal in efficiency that is no problem.

As a final disclaimer, I know my way around in ruby/rails, but am fairly green in SQL.

Thanks for the help!

Answer 1

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp;

SET search_path='tmp';


CREATE TABLE instrument
        ( id INTEGER NOT NULL PRIMARY KEY
        , zname varchar
        );
INSERT INTO instrument(id, zname) VALUES
(1, 'instrument_1'), (2, 'instrument_2')
, (3, 'instrument_3'), (4, 'instrument_4');

CREATE TABLE piece
        ( id INTEGER NOT NULL PRIMARY KEY
        , zname varchar
        );
INSERT INTO piece(id, zname) VALUES
(1, 'piece_1'), (2, 'piece_2'), (3, 'piece_3'), (4, 'piece_4');

CREATE TABLE has_part
        ( piece_id INTEGER NOT NULL
        , instrument_id INTEGER NOT NULL
        , PRIMARY KEY (piece_id,instrument_id)
        );

INSERT INTO has_part(piece_id,instrument_id) VALUES
(1,1), (1,2), (1,3)
, (2,1), (2,2), (2,3), (2,4)
, (3,1), (3,3), (3,4)
, (4,2)
        ;

The pure sql (not the double negation NOT EXISTS , NOT IN():

SELECT zname
FROM piece pp
WHERE NOT EXISTS (
        SELECT * FROM has_part nx
        WHERE nx.piece_id = pp.id
        AND nx.instrument_id NOT IN (1,2,3)
        )
        ;