Validate that there is exactly one subject present

- Update the database constraint
- Adjust the existing customer validator on ExperimentSubject :base
- Add private non_nil_subjects helper method to model

app/models/experiment_subject.rb

@@ -10,15 +10,19 @@ class ExperimentSubject < ApplicationRecord
 
   validates :experiment, presence: true
   validates :variant, presence: true
-  validate :must_have_at_least_one_subject
+  validate :must_have_one_subject_present
 
   enum variant: { GROUP_CONTROL => 0, GROUP_EXPERIMENTAL => 1 }
 
   private
 
-  def must_have_at_least_one_subject
-    if [user, group, project].all?(&:blank?)
-      errors.add(:base, s_("ExperimentSubject|Must have at least one of User, Group, or Project."))
+  def must_have_one_subject_present
+    if non_nil_subjects.length != 1
+      errors.add(:base, s_("ExperimentSubject|Must have exactly one of User, Group, or Project."))
     end
   end
+
+  def non_nil_subjects
+    @non_nil_subjects ||= [user, group, project].reject(&:blank?)
+  end
 end

db/migrate/20201110221400_create_experiment_subjects.rb

@@ -13,9 +13,9 @@ class CreateExperimentSubjects < ActiveRecord::Migration[6.0]
       t.timestamps_with_timezone null: false
     end
 
-    # Require at least one of user_id, group_id, or project_id to be NOT NULL
+    # Require exactly one of user_id, group_id, or project_id to be NOT NULL
     execute <<-SQL
-      ALTER TABLE experiment_subjects ADD CONSTRAINT chk_at_least_one_subject CHECK (NOT ROW(user_id, group_id, project_id) IS NULL);
+      ALTER TABLE experiment_subjects ADD CONSTRAINT chk_has_one_subject CHECK (num_nonnulls(user_id, group_id, project_id) = 1);
     SQL
   end
 

db/structure.sql

@@ -12147,7 +12147,7 @@ CREATE TABLE experiment_subjects (
     variant smallint DEFAULT 0 NOT NULL,
     created_at timestamp with time zone NOT NULL,
     updated_at timestamp with time zone NOT NULL,
-    CONSTRAINT chk_at_least_one_subject CHECK ((NOT (ROW(user_id, group_id, project_id) IS NULL)))
+    CONSTRAINT chk_has_one_subject CHECK ((num_nonnulls(user_id, group_id, project_id) = 1))
 );
 
 CREATE SEQUENCE experiment_subjects_id_seq

locale/gitlab.pot

@@ -11307,7 +11307,7 @@ msgstr ""
 msgid "Experienced"
 msgstr ""
 
-msgid "ExperimentSubject|Must have at least one of User, Group, or Project."
+msgid "ExperimentSubject|Must have exactly one of User, Group, or Project."
 msgstr ""
 
 msgid "Expiration"

spec/models/experiment_subject_spec.rb

@@ -12,36 +12,43 @@ RSpec.describe ExperimentSubject, type: :model do
 
   describe 'validations' do
     it { is_expected.to validate_presence_of(:experiment) }
-  end
-
-  describe 'validate must_have_at_least_one_subject' do
-    let(:experiment_subject) { build(:experiment_subject, user: nil, group: nil, project: nil) }
-
-    it 'fails if user, group, & project are blank' do
-      expect(experiment_subject).not_to be_valid
-      expect(experiment_subject.errors[:base]).to include("Must have at least one of User, Group, or Project.")
-    end
-
-    it 'passes when user is present' do
-      experiment_subject.user = build(:user)
-      expect(experiment_subject).to be_valid
-    end
-
-    it 'passes when group is present' do
-      experiment_subject.group = build(:group)
-      expect(experiment_subject).to be_valid
-    end
-
-    it 'passes when project is present' do
-      experiment_subject.project = build(:project)
-      expect(experiment_subject).to be_valid
-    end
 
-    it 'passes when multiple subjects are present' do
-      experiment_subject.user = build(:user)
-      experiment_subject.group = build(:group)
-      experiment_subject.project = build(:project)
-      expect(experiment_subject).to be_valid
+    describe 'must_have_one_subject_present' do
+      let(:experiment_subject) { build(:experiment_subject, user: nil, group: nil, project: nil) }
+      let(:error_message) { 'Must have exactly one of User, Group, or Project.' }
+
+      it 'fails when no subject is present' do
+        expect(experiment_subject).not_to be_valid
+        expect(experiment_subject.errors[:base]).to include(error_message)
+      end
+
+      it 'passes when user subject is present' do
+        experiment_subject.user = build(:user)
+        expect(experiment_subject).to be_valid
+      end
+
+      it 'passes when group subject is present' do
+        experiment_subject.group = build(:group)
+        expect(experiment_subject).to be_valid
+      end
+
+      it 'passes when project subject is present' do
+        experiment_subject.project = build(:project)
+        expect(experiment_subject).to be_valid
+      end
+
+      it 'fails when more than one subject is present', :aggregate_failures do
+        # two subjects
+        experiment_subject.user = build(:user)
+        experiment_subject.group = build(:group)
+        expect(experiment_subject).not_to be_valid
+        expect(experiment_subject.errors[:base]).to include(error_message)
+
+        # three subjects
+        experiment_subject.project = build(:project)
+        expect(experiment_subject).not_to be_valid
+        expect(experiment_subject.errors[:base]).to include(error_message)
+      end
     end
   end
 end