Add unique constraint to viz.permission_id (1:1 relation)

db/migrate/20160412144305_add_unique_constraint_to_permission_id.rb

@@ -0,0 +1,13 @@
+Sequel.migration do
+  up do
+    alter_table :visualizations do
+      add_unique_constraint :permission_id
+    end
+  end
+
+  down do
+    alter_table :visualizations do
+      drop_constraint :visualizations_permission_id_key
+    end
+  end
+end