validate the foreign_key instead of the relation

app/models/gpg_signature.rb

@@ -8,7 +8,7 @@ class GpgSignature < ActiveRecord::Base
   belongs_to :gpg_key
 
   validates :commit_sha, presence: true
-  validates :project, presence: true
+  validates :project_id, presence: true
   validates :gpg_key_primary_keyid, presence: true
 
   def gpg_key_primary_keyid

spec/models/gpg_signature_spec.rb

@@ -9,7 +9,7 @@ RSpec.describe GpgSignature do
   describe 'validation' do
     subject { described_class.new }
     it { is_expected.to validate_presence_of(:commit_sha) }
-    it { is_expected.to validate_presence_of(:project) }
+    it { is_expected.to validate_presence_of(:project_id) }
     it { is_expected.to validate_presence_of(:gpg_key_primary_keyid) }
   end