How to access NEW or OLD field given only the field's name?

Hi all!

Cross-posted from
https://dba.stackexchange.com/questions/61271/how-to-access-new-or-old-field-given-only-the-fields-name

I'm writing a validation trigger. The trigger must validate that the sum of an array equals another field. Since I have
manyinstances of this validation, I want to write a single procedure and create multiple triggers, each with a
differentset of fields to check. 

For example, I have the following schema:

CREATE TABLE daily_reports(
  start_on date
, show_id uuid
, primary key(start_on, show_id)

-- _graph are hourly values, while _count is total for the report
, impressions_count bigint not null
, impressions_graph bigint[] not null

-- interactions_count, interactions_graph
-- twitter_interactions_count, twitter_interactions_graph
);

The validation must confirm that impressions_count = sum(impressions_graph).

I'm stuck because I don't know how to dynamically access a field from NEW from within plpgsql:

CREATE FUNCTION validate_sum_of_array_equals_other() RETURNS TRIGGER AS $$
DECLARE
  total bigint;
  array_sum bigint;
BEGIN
-- TG_NARGS = 2
-- TG_ARGV[0] = 'impressions_count'
-- TG_ARGV[1] = 'impressions_graph'

-- How to access impressions_count and impressions_graph from NEW?

RETURN NEW;
END
$$ LANGUAGE plpgsql;

CREATE TRIGGER validate_daily_reports_impressions
ON daily_reports BEFORE INSERT OR UPDATE
FOR EACH ROW EXECUTE
  validate_sum_of_array_equals_other('impressions_count', 'impressions_graph');

I tried http://www.postgresql.org/docs/9.1/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN by doing
EXECUTE'SELECT $1 FROM NEW' INTO total USING TG_ARGV[0], but PL/PGsql complains that NEW is an unknown relation. 

I am specifically targeting PostgreSQL 9.1.

Thanks for any hints!
François Beausoleil