Simple Form - How to Use Relational Data as Label Method

I'm having problems with relational data in simple form. So far "collection" is working well in the form, but I couldn't figure it out how to use "label_method" properly:

<%= f.input :participation_id, collection: Participation.where(user_id: current_user.id), as: :select, label_method: "????", label: 'Choose' %>

I would like to use the "examination" name from my "examination" model. I can retreive it from the model like this in the console:

p = Participation.where(user_id: 1)
p.first.examination.name

But as you might guess it is returning multiple values, because I have 3 different participations for the user with ID 1, and each participation has a different examination name.

How can I retreive each examination's name for each participation in the "label_method"?

Thanks.

new.html.erb =>

<%= simple_form_for(@order, html:{class: "well"}, :url => "https://somewhere.com", :method => :post) do |f| %>
   <%= f.input :participation_id, collection: Participation.where(user_id: current_user.id), as: :select, label_method: "????", label: 'Choose' %>
   <%= f.input :user_id, as: :hidden, input_html: { value: current_user.id }  %>
   <%= f.button :submit %>
<% end %>

My Models =>

class Participation < ActiveRecord::Base
  belongs_to :user
  belongs_to :examination
  has_one :order
end

class Order < ActiveRecord::Base
  belongs_to :user
  belongs_to :participation
end

class Examination < ActiveRecord::Base
  has_many :participations
  has_many :users, :through => :participations
end

Database Structure =>

create_table "examinations", force: true do |t|
    t.string   "name"
    t.string   "shortname"
    t.datetime "exam_date"
end

create_table "participations", force: true do |t|
    t.integer  "user_id"
    t.integer  "examination_id"
    t.string   "language_preference"
    t.string   "exam_center_preference"
end

create_table "orders", force: true do |t|
    t.string   "first_name"
    t.string   "last_name"
    t.integer  "participation_id"
    t.integer  "user_id"
end