PostgreSQL: How to concatenate rows [duplicate]

So, I have a pretty unconventional problem. I would like to be able to concatenate rows with the same ID into one big row. To illustrate my problem let me provide an example. Here is the query:

SELECT b.id AS "ID",
       m.content AS "Conversation"
FROM bookings b 
INNER JOIN conversations c on b.id = c.conversable_id AND c.conversable_type = 'Booking'
INNER JOIN messages m on m.conversation_id = c.id
WHERE b.state IS NOT NULL
GROUP BY 1,2
LIMIT 1000;

And here is the output:

ID     **Conversation
1223    "blah, blah, blah, blah"
1223    " ... blaaaah, blah.."
1223    "more blaaah"
1223    "last blah"
5000    "new id, with more blah"
5000    "and the blah continues"

Is there a way to concatenate the conversation rows into one aggregate row while keeping the ID?

Like this:

ID     Conversation
1223    "blah, blah, blah, blah, ... blaaaah blah.. more blaaah, last blah"
5000    "new id, with more blah and the blah continues"

I am sure there is an efficient way to do this. I just can't figure it out on my own.