Simple form radio button

In your controller action, add this line

  def actionname
    @types = ModelName.select(:diettype).distinct
    ..
  end

where,

actionname is the action which is rendering your view.

ModelName is the name of your model which has diettype field in it.

In your view, replace

<%= f.collection_radio_buttons :diettype, [[vegan, 'vegan'] ,[vegetarian, 'vegetarian']]%>

with

<%= f.collection_radio_buttons :diettype, @types, :diettype, :diettype %>

EDIT:

<%= f.collection_radio_buttons :diettype, @types, :diettype, :diettype %>

The above line means:

Create a collection of radio buttons where,

1st :diettype : variable would be set when you select a radio button

@types : this collection being passed

2nd :diettype : the value that is being selected

3rd :diettype : the display text beside the button

EDIT2

As you specified that you need a static collection and you are not taking any values from database :

Simply add the following line in view, no need to change the controller :

<%= f.collection_radio_buttons :name, [['vegan', 'vegan'] ,['vegetarian', 'vegetarian']],:first, :last %> 

This is the simplest solution for adding radio buttons with Simple Form that works great for me.

 <%= f.input :diettype, as: :radio_buttons, collection: ['Vegan', 'Vegetarian'] %>