Rails: Unable to save nested attributes - ruby-on-rails

Question

I have two models: User and WritingSample. A user can have many writing samples. I have a page with a form with extra JS for adding additional writing sample fields -- however, I've disabled this for the time being and I'm trying to get it working with just the one.
What I want to happen: the user types the url of a writing sample into the form and this is stored as a WritingSample row.
The form seems to be working. But I'm clearly not doing something right.
When I submit the form, I can see that safe_params contains:
"writing_samples_attributes"=><ActionController::Parameters {"0"=><ActionController::Parameters {"url"=>"foo"} permitted: true>}
But it looks like User wants an id:
> User
User(id: integer, name: string, writing_sample_id: integer)
User.new(safe_params) returns:
<User id: nil, name: "joe", created_at: nil, updated_at: nil, writing_sample_id: nil>
The new user isn't created, and I get the error:
user.errors.first
[:"writing_samples.user", "must exist"]
What steps do I need to take to store the submitted data in the :url field?
WritingSample:
class WritingSample < ApplicationRecord
belongs_to :user
end
User:
class User < ApplicationRecord
has_many :writing_samples
accepts_nested_attributes_for :writing_samples
end
Schema:
create_table "users", force: :cascade do |t|
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "writing_sample_id"
t.index ["writing_sample_id"], name: "index_users_on_writing_sample_id"
end
create_table "writing_samples", force: :cascade do |t|
t.string "name"
t.integer "user_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.string "url"
t.index ["user_id"], name: "index_writing_samples_on_user_id"
end
My controller:
def create
user = User.new(safe_params)
byebug
if user.save
redirect_to '/thanks'
else
byebug
redirect_to new_creator_path
end
end
private
def safe_params
params.require(:user).permit(:name, :writing_samples_attributes => [:url])
end
The form:
<%= form_for(#user, url: creators_path) do |f| %>
<%= f.label :name %>
<%= f.label 'Writing samples' %>
<%= f.text_field :name, class: 'form-control' %>
<%= f.fields_for :writing_samples do |sample_f| %>
<%= sample_f.text_field :url %>
<% end %>
<% end %>