Change migration to emit a warning if data exists

If there is related data, it is not deleted but the user is given a
warning to allow them to migrate to ckanext-showcase and how to manually
delete the tables.

ckan/migration/versions/083_remove_related_items.py

@@ -1,5 +1,24 @@
 
+WARNING = """
+
+WARNING: The 'related' tables were not deleted as they currently contain data.
+Once you have archived the existing data or migrated the data to
+ckanext-showcase, you can safely delete the 'related' and 'related_dataset'
+tables using:
+
+    psql ckan_default -c 'BEGIN; DROP TABLE related_dataset; \\
+    DROP TABLE related; COMMIT;'
+
+"""
+
+
 def upgrade(migrate_engine):
+    existing = migrate_engine.execute("SELECT COUNT(*) FROM related;")\
+        .fetchone()
+    if existing[0] > 0:
+        print WARNING
+        return
+
     migrate_engine.execute('''
 BEGIN;
 DROP TABLE related_dataset;