Question

Background

In a PostgreSQL 9.0 database, there are various tables that have many-to-many relationships. The number of those relationships must be restricted. A couple of example tables include:

CREATE TABLE authentication (
  id bigserial NOT NULL, -- Primary key
  cookie character varying(64) NOT NULL, -- Authenticates the user with a cookie
  ip_address character varying(40) NOT NULL -- Device IP address (IPv6-friendly)
)

CREATE TABLE tag_comment (
  id bigserial NOT NULL, -- Primary key
  comment_id bigint, -- Foreign key to the comment table
  tag_name_id bigint -- Foreign key to the tag name table
)

Different relationships, however, have different limitations. For example, in the authentication table, a given ip_address is allowed 1024 cookie values; whereas, in the tag_comment table, each comment_id can have 10 associated tag_name_ids.

Problem

Currently, a number of functions have these restrictions hard-coded; scattering the limitations throughout the database, and preventing them from being changed dynamically.

Question

How would you impose a maximum many-to-many relationship limit on tables in a generic fashion?

Idea

Create a table to track the limits:

CREATE TABLE imposed_maximums (
  id serial NOT NULL,
  table_name  character varying(128) NOT NULL,
  column_group character varying(128) NOT NULL,
  column_count character varying(128) NOT NULL,
  max_size INTEGER
)

Establish the restrictions:

INSERT INTO imposed_maximums
  (table_name, column_group, column_count, max_size) VALUES
  ('authentication', 'ip_address', 'cookie', 1024);
INSERT INTO imposed_maximums
  (table_name, column_group, column_count, max_size) VALUES
  ('tag_comment', 'comment_id', 'tag_id', 10);

Create a trigger function:

CREATE OR REPLACE FUNCTION impose_maximum()
  RETURNS trigger AS
$BODY$
BEGIN
  -- Join this up with imposed_maximums somehow?
  select
    count(1)
  from
    -- the table name
  where
    -- the group column = NEW value to INSERT;

  RETURN NEW;
END;

Attach the trigger to every table:

CREATE TRIGGER trigger_authentication_impose_maximum
  BEFORE INSERT
  ON authentication
  FOR EACH ROW
  EXECUTE PROCEDURE impose_maximum();

Obviously it won’t work as written… is there a way to make it work, or otherwise enforce the restrictions such that they are:

• in a single location; and
• not hard-coded?

Thank you!

Answer 1

I’ve been doing a similar type of generic triggers.
The most tricky part is to get the value entry in the NEW record based on the column name.

I’m doing it the following way:

• convert NEW data into array;
• find the attnum of the column and use it as an index for the array.

This approach works as long as there’re no commas in the data 🙁 I don’t know of other ways how to convert NEW or OLD variables into the array of values.

The following function might help:

CREATE OR REPLACE FUNCTION impose_maximum() RETURNS trigger AS $impose_maximum$
DECLARE
  _sql  text;
  _cnt  int8;
  _vals text[];
  _anum int4;
  _im   record;

BEGIN
 _vals := string_to_array(translate(trim(NEW::text), '()', ''), ',');

 FOR _im IN SELECT * FROM imposed_maximums WHERE table_name = TG_TABLE_NAME LOOP
  SELECT attnum INTO _anum FROM pg_catalog.pg_attribute a
    JOIN pg_catalog.pg_class t ON t.oid = a.attrelid
   WHERE t.relkind = 'r' AND t.relname = TG_TABLE_NAME
     AND NOT a.attisdropped AND a.attname = _im.column_group;

  _sql := 'SELECT count('||quote_ident(_im.column_count)||')'||
          ' FROM '||quote_ident(_im.table_name)||
          ' WHERE '||quote_ident(_im.column_group)||' = $1';

  EXECUTE _sql INTO _cnt USING _vals[_anum];

  IF _cnt > CAST(_im.max_size AS int8) THEN
    RAISE EXCEPTION 'Maximum of % hit for column % in table %(%=%)',
      _im.max_size, _im.column_count,
      _im.table_name, _im.column_group, _vals[_anum];
  END IF;
 END LOOP;

 RETURN NEW;
END; $impose_maximum$ LANGUAGE plpgsql;

This function will check for all conditions defined for a given table.