add price feed trigger

db/migrations/20180911215603_add_price_feed_trigger.down.sql

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+DROP TRIGGER notify_pricefeeds ON maker.price_feeds;

db/migrations/20180911215603_add_price_feed_trigger.up.sql

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+CREATE OR REPLACE FUNCTION notify_pricefeed() RETURNS trigger AS $$
+BEGIN
+  PERFORM pg_notify(
+    CAST('postgraphile:price_feed' AS text),
+    row_to_json(NEW)::text);
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER notify_pricefeeds
+  AFTER INSERT ON maker.price_feeds
+  FOR EACH ROW
+  EXECUTE PROCEDURE notify_pricefeed();