one_to_many.rb

# app/models/author.rb
class Author < ActiveRecord::Base
  has_many :interests
end

# app/models/interest.rb
class Interest < ActiveRecord::Base
  belongs_to :author
end

# app/controllers/authors_controller.rb
...
private
    def set_author
      @author = Author.find(params[:id])
    end

    def author_params
      params.require(:author).permit(:name, :interest_id)
    end
...

# app/views/authors/_form.html.erb
<%= form_for(@author) do |f| %>
  <% if @author.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@author.errors.count, "error") %> prohibited this author from being saved:</h2>

      <ul>
      <% @author.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br>
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :author_id %><br>
    <%= f.collection_select(:interest_id, Interest.all, :id, :name, prompt: true) %>
  </div>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>