Trigger to update a different table - sql

Question

Using Postgres 9.4, I have 2 tables streams and comment_replies. I am trying to do is update the streams.comments count each time a new comment_replies is inserted to keep track of the number of comments a particular stream has. I am not getting any errors but when I try to create a new comment it gets ignored.
This is how I am setting up my trigger. stream_id is a foreign key, so every stream_id will correspond to a streams.id which is the primary key of the streams table. I have been looking at this example: Postgres trigger function, but haven't been able to get it to work.
CREATE TABLE comment_replies (
id serial NOT NULL PRIMARY KEY,
created_on timestamp without time zone,
comments text,
profile_id integer,
stream_id integer
);
The trigger function:
CREATE OR REPLACE FUNCTION "Comment_Updates"()
RETURNS trigger AS
$BODY$BEGIN
update streams set streams.comments=streams.comments+1
where streams.id=comment_replies_streamid;
END$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
And the trigger:
CREATE TRIGGER comment_add
BEFORE INSERT OR UPDATE
ON comment_replies
FOR EACH ROW
EXECUTE PROCEDURE "Comment_Updates"();
How can I do this?

Answer 1

There are multiple errors. Try instead:
CREATE OR REPLACE FUNCTION comment_update()
RETURNS trigger AS
$func$
BEGIN
UPDATE streams s
SET streams.comments = s.comments + 1
-- SET comments = COALESCE(s.comments, 0) + 1 -- if the column can be NULL
WHERE s.id = NEW.streamid;
RETURN NEW;
END
$func$ LANGUAGE plpgsql;
CREATE TRIGGER comment_add
BEFORE INSERT OR UPDATE ON comment_replies -- on UPDATE, too? Really?
FOR EACH ROW EXECUTE PROCEDURE comment_update();
You need to consider DELETE as well if that is possible. Also if UPDATE can change stream_id. But why increase the count for every UPDATE? This looks like another error to me.
It's a syntax error to table-qualify the target column in the SET clause of UPDATE.
You need to return NEW in a BEFORE trigger unless you want to cancel the INSERT / UPDATE.
Or you make it an AFTER trigger, which would work for this, too.
You need to reference NEW for the stream_id of the current row (which is automatically visible inside the trigger function.
If streams.comments can be NULL, use COALESCE.
And rather use unquoted, legal, lower-case identifiers.