string_agg No function matches the given name

I have relational database and would like to use `string_agg()` since it seems fit to my need.<br><br>
**I want :**<br>

product_id | quiz_id
-----------+----------
1 | 1,6
2 | 2,7
3 | 3,8
4 | 4

**Here is my database.**

select quiz_id , product_id, lastmodified from dugong.quiz;
quiz_id | product_id | lastmodified
---------+------------+-------------------------------
1 | 1 | 2015-11-11 14:46:55.619162+07
2 | 2 | 2015-11-11 14:46:55.619162+07
3 | 3 | 2015-11-11 14:46:55.619162+07
4 | 4 | 2015-11-11 14:46:55.619162+07
5 | 5 | 2015-11-11 14:46:55.619162+07
6 | 1 | 2015-11-11 14:46:55.619162+07
7 | 2 | 2015-11-11 14:46:55.619162+07
8 | 3 | 2015-11-11 14:46:55.619162+07

**My attempt :** <br>
Refer to document.

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select product_id , string_agg(quiz_id, ',' order by lastmodified) from dugong.quiz;
ERROR: function string_agg(integer, unknown) does not exist
LINE 1: select product_id , string_agg(quiz_id, ',' order by lastmod...
^
HINT: No function matches the given name and argument types. You might need to add explicit type casts.
<br>
**Postgres version :** <br>
PostgresApp 9.4.4.1

**Update :** <br>
@code-monk
It still error.

select product_id , string_agg(quiz_id::int, ',' order by lastmodified) from dugong.quiz;
ERROR: function string_agg(integer, unknown) does not exist
LINE 1: select product_id , string_agg(quiz_id::int, ',' order by la...
^
HINT: No function matches the given name and argument types. You might need to add explicit type casts.


**Question :**<br>
What is wrong with my query?

Try this:


select product_id , string_agg(quiz_id::character varying, ',' order by lastmodified)
from quiz group by product_id;


`String_agg` function works with String values only ,You are getting the error because `quiz_id` is integer.

I have converted it to `character varying` and added group by for grouping the data product ID wise.

SQL Fiddle Example:http://sqlfiddle.com/#!15/9dafe/1