Unable to traverse one-to-one relationship in Rails - relationship

Question

I've got an application where each user has one keyholder, who is allowed to access their account. These are separate models. I've set up the one-to-one relationship within the models, however when I try and use this within the views, I am getting the error of 'undefined method 'id' for nil:NilClass. I've used relationships and traversed them many times before, but I don't understand why this is happening, so please can someone explain?
The only thing that I have done differently from normal is to use Devise to generate the keyholder model rather than scaffolding it as I normally would.
Thanks!
user.rb:
has_one :keyholder
keyholder.rb:
belongs_to :user, dependent: :destroy
And neither of these views work:
keyholders/index.html.erb
<% #keyholders.each do |keyholder| %>
<tr>
<td><%= keyholder.id %></td>
<td><%= keyholder.username %></td>
<td><%= keyholder.user.id %></td>
<td><%= link_to 'Show', keyholder %></td>
<td><%= link_to 'Edit', edit_keyholder_path(keyholder) %></td>
<td><%= link_to 'Destroy', keyholder, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
or keyholder/show.html.erb
<p>
<strong>ID:</strong>
<%= #keyholder.id %>
</p>
<p>
<strong>Username:</strong>
<%= #keyholder.username %>
</p>
<p>
<strong>Keyholder for:</strong>
<%= #keyholder.user.first_name %> <%= #keyholder.user.last_name %>
</p>
<p>
<strong>User ID:</strong>
<%= #keyholder.user.id %>
</p>
Controller code:
def index
#keyholders = Keyholder.all
end
def show
#keyholder = Keyholder.find(params[:id])
end

Answer 1

I tried everything to check where any errors could be - for example checking that both the keyholder's user_id and the user's keyholder_id were both integers etc.
In case anyone is reading this and wants the code to do so, it is:
#model.column_for_attribute('column_name').type
I didn't manage to fix this in the end - luckily I was just starting out with this app, so started again and scaffolded both models before adding Devise to them, and it now works fine having done everything else the same.