Support external ID when importing issues (#28213).

Patch by Gregor Schmidt and Marius BALTEANU.

git-svn-id: http://svn.redmine.org/redmine/trunk@18285 e93f8b46-1217-0410-a6f0-8f06a7374b81

db/migrate/20190510070108_add_unique_id_to_import_items.rb

@@ -0,0 +1,8 @@
+class AddUniqueIdToImportItems < ActiveRecord::Migration[5.2]
+  def change
+    change_table :import_items do |t|
+      t.string "unique_id"
+      t.index ["import_id", "unique_id"]
+    end
+  end
+end