What is the proper way to check for a foreign key with Rails 5.2.4 / rspec 3.12?

In my application, I define that a Playground belongs to a Owner. In the create table, the owner foreign key is defined by:

t.belongs_to :owner, null: false, comment: "All managed objects have a owner"

As well as in the app/models/playground.rb

class Playground < ApplicationRecord
  validates :code,        presence: true, 
                          uniqueness: { case_sensitive: false }, 
                          length: { maximum: 32 }
  validates :name,        presence: true
  belongs_to :owner,      class_name: "User",         foreign_key: "owner_id"   
end

Which corresponds to the specs I wrote in spec/models/playground_spec.rb:

require 'rails_helper'

RSpec.describe Playground, type: :model do

  describe 'Validations'
  subject {FactoryBot.build(:playground)}
    it { should validate_presence_of(:code) }
    it { should validate_uniqueness_of(:code).case_insensitive }
    it { should validate_length_of(:code).is_at_most(32)}
    it { should validate_presence_of(:name) }
    it { should validate_presence_of(:owner) }

  describe 'It can be created'
  it 'has a valid factory' do
    expect(build(:playground)).to be_valid
  end
  it 'is invalid without a code' do
    expect(build(:playground, code: nil)).to_not be_valid
  end

end

Nevertheless, when I run the test, I get the following error:

>rspec spec/models/playground_spec.rb

Failures:

  1) Playground is expected to validate that :owner cannot be empty/falsy
     Failure/Error: it { should validate_presence_of(:owner) }

       Expected Playground to validate that :owner cannot be empty/falsy, but
       this could not be proved.
         After setting :owner to ‹nil›, the matcher expected the Playground to
         be invalid, but it was valid instead.
     # ./spec/models/playground_spec.rb:48:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:55:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:54:in `block (2 levels) in <top (required)>'

Finished in 4.27 seconds (files took 21.09 seconds to load)
13 examples, 1 failure

Failed examples:

rspec ./spec/models/playground_spec.rb:48 # Playground is expected to validate that :owner cannot be empty/falsy

What is the proper way to make sure my model actually checks for the existence of the foreign key ?