FIX: drafts are unique by draft_key and user_id

Previously our index was non unique, causing situations where a user could
have multiple drafts stored in the table for the same exact entity.

This does not properly reflect reality and needed to change as in certain
cases duplicate drafts could be created causing internal data inconsistency

app/models/draft.rb

@@ -282,5 +282,5 @@ end
 #
 # Indexes
 #
-#  index_drafts_on_user_id_and_draft_key  (user_id,draft_key)
+#  index_drafts_on_user_id_and_draft_key  (user_id,draft_key) UNIQUE
 #

db/migrate/20191108000414_add_unique_index_to_drafts.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexToDrafts < ActiveRecord::Migration[6.0]
+  def up
+
+    execute <<~SQL
+      DELETE FROM drafts d1
+      USING (
+        SELECT MAX(id) as id, draft_key, user_id
+        FROM drafts
+        GROUP BY draft_key, user_id
+        HAVING COUNT(*) > 1
+      ) d2
+      WHERE
+        d1.draft_key = d2.draft_key AND
+        d1.user_id = d2.user_id AND
+        d1.id <> d2.id
+    SQL
+
+    remove_index :drafts, [:user_id, :draft_key]
+    add_index :drafts, [:user_id, :draft_key], unique: true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end