instead of defining a new instance var on the controller only to use it once in a view, you can use helpers such as options_from_collection_for_select. Also please preselect value in a select dropdown (important to render a form with previously selected value after a validation error)

e.g. a select for designation in the change_type editing form looks like this:

  <%= f.select :designation_id,
    options_from_collection_for_select(@designations, :id, :name, @change_type && @change_type.designation_id)
  %>

For the purpose / unit dropdown we also need to include the blank option.
The third arg of options_for... is the value to preselect, if present.