New Feature: Watch Listings (#215)

* adding new feature: watch listings for changes

* adding todo for watch feature

* sort by watch

lib/services/storage/migrations/sql/3.changeset-for-listings.js

@@ -0,0 +1,8 @@
+// Migration: Adding a changeset field to the listings table in preparation for
+// a price watch feature
+
+export function up(db) {
+  db.exec(`
+    ALTER TABLE listings ADD COLUMN change_set jsonb;
+  `);
+}

lib/services/storage/migrations/sql/4.watch-list.js

@@ -0,0 +1,15 @@
+// Migration: Adding a new table to store if somebody "watches" (a.k.a favorite) a listing
+
+export function up(db) {
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS watch_list
+    (
+      id         TEXT PRIMARY KEY,
+      listing_id TEXT NOT NULL,
+      user_id    TEXT NOT NULL,
+      FOREIGN KEY (listing_id) REFERENCES listings (id) ON DELETE CASCADE,
+      FOREIGN KEY (user_id)    REFERENCES users    (id) ON DELETE CASCADE
+    );
+    CREATE UNIQUE INDEX IF NOT EXISTS idx_watch_list ON watch_list (listing_id, user_id);
+  `);
+}