An enum field can be displayed as `select` or as `radio_buttons`.

Here's 2 ways of `select`, with a blank input available because of `include_blank` and `prompt`.

```erb
<%= f.select :gender, User::GENDER.keys, include_blank: true %>

<%= collection_select 'user' ,'gender', User::GENDER, :first, :first, prompt: "Select something" %>
```