feat(url-clicks): migration script

src/server/db_migrations/20201207_synchronise_url_clicks.sql

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+-- This SQL file is used to populate the url_clicks table with data from the
+-- url_table, and also add the necessary hooks such that they will always be
+-- in sync. These include 1) when a url_table row's click is changed, and
+-- 2) when a new url_table row is created.
+BEGIN TRANSACTION;
+
+INSERT INTO "url_clicks" ("shortUrl", "clicks", "createdAt", "updatedAt")
+SELECT "shortUrl", "clicks", "createdAt", "updatedAt" from urls;
+
+CREATE OR REPLACE FUNCTION update_clicks()
+RETURNS TRIGGER AS $$
+BEGIN
+    UPDATE url_clicks
+    SET "clicks" = NEW."clicks"
+    WHERE url_clicks."shortUrl" = NEW."shortUrl";
+    RETURN NEW;
+END; $$ LANGUAGE PLPGSQL;
+
+CREATE TRIGGER url_table_click_incremented
+    AFTER UPDATE of "clicks" on urls
+    FOR EACH ROW
+    WHEN (OLD."clicks" <> NEW."clicks")
+    EXECUTE PROCEDURE update_clicks();
+
+CREATE OR REPLACE FUNCTION create_clicks()
+    RETURNS TRIGGER AS $$
+BEGIN
+    INSERT INTO url_clicks ("shortUrl", "clicks", "createdAt", "updatedAt")
+    VALUES (NEW."shortUrl", NEW."clicks", NEW."createdAt", NEW."updatedAt");
+    RETURN NEW;
+END; $$ LANGUAGE PLPGSQL;
+
+CREATE TRIGGER url_created
+    AFTER INSERT on urls
+    FOR EACH ROW
+    EXECUTE PROCEDURE create_clicks();
+
+COMMIT;