Find values in specific column of unknown table in PostgreSQL from information_schema

Question

Using pl/pgsql I've hacked together an update function to run on a bunch of tables that I need to change but I don't know all the names of. What I do know is THEY ALL have a primary key (which could be named anything) and a column specifically called 'metadata'. For example of one of these 'unknown' named tables might look like this:

CREATE TABLE test
(
    gid SERIAL NOT NULL PRIMARY KEY,
    item varchar,
    metadata varchar,
    z_order int,
    foobar int
);

and I'm trying to return for each row I update (created by an BEFORE UPDATE trigger):

history | action    | table_name | column_name| pkey_id | metadata    |
1       | UPDATE    | test       | item       |       1 |          y  |
_____________________________________________________________________ |
2       | UPDATE    | test       | z_order    |       1 |          y  |
_____________________________________________________________________
3       | UPDATE    | test       | foobar     |       1 |          y  |

However, I am having problems calling the specific column_name "metadata" in my query...for id'ing the pkey_id (which might have any column name) I used information_schema:

    SELECT
        INTO pkey_column_name
        kcu.column_name
    FROM
        information_schema.table_constraints AS tc
        JOIN information_schema.key_column_usage AS kcu
        ON tc.constraint_name = kcu.constraint_name
        JOIN information_schema.constraint_column_usage AS ccu
        ON ccu.constraint_name = tc.constraint_name
    WHERE
        constraint_type = 'PRIMARY KEY'
    AND
        ccu.table_name=tg_relname;

 /// more code ///
  FOR each_entity IN
     EXECUTE 'SELECT text((' || QUOTE_NULLABLE(OLD) || '::"' || QUOTE_IDENT(tg_table_schema) || '"."' || QUOTE_IDENT(tg_relname) || '")."'  || QUOTE_IDENT(pkey_column_name) || '")
                AS pk_val, 
/// more code ///

But...my attempts to get the specific column_name "metadata" into my execute query have failed. Ideally I would like to select it by name using attname...but I'm not having much luck...I have a list of each column ('each_column') names (including 'metadata') but no way to call it in the EXECUTE statement...

FOR each_column IN
    SELECT
        attname as column_name
    FROM
        pg_attribute
    WHERE
        attrelid =
        (
            SELECT
                oid --  'oid' is the internal Postgresql primary key used for various system tables
            FROM
                pg_class
            WHERE
                relname = tg_relname -- 'tg_relname' is a special Trigger variable that gives the name of the table invoking the trigger
        )
    AND attnum > 0 -- returns no system columns 
LOOP

Answer 1

This is what I came up with (thanks Denis for the advice)~

http://www.sqlfiddle.com/#!15/64e41/2/0

Answer 2

Instead of:

FOR column_name IN SELECT ...

Use:

SELECT ARRAY(SELECT ...) INTO variable

Then use array_to_string() with adequate quote_ident() calls, or if you need it conditionally or with where clauses, the same with something like:

array_to_string((select array(... from unnest(variable) cols where ...)), ',')

This should allow you to build any number of select or where clauses (using ' AND ' instead of ',') as needed, with any number of potential conditions on them.