Replace optional: false with presence validation

The follow up of the following discussion:

https://gitlab.com/gitlab-org/gitlab
/-/merge_requests/24360#note_282247872

app/models/prometheus_alert.rb

@@ -9,9 +9,9 @@ class PrometheusAlert < ApplicationRecord
     gt: ">"
   }.freeze
 
-  belongs_to :environment, optional: false, validate: true, inverse_of: :prometheus_alerts
-  belongs_to :project, optional: false, validate: true, inverse_of: :prometheus_alerts
-  belongs_to :prometheus_metric, optional: false, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :environment, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :project, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :prometheus_metric, validate: true, inverse_of: :prometheus_alerts
 
   has_many :prometheus_alert_events, inverse_of: :prometheus_alert
   has_many :related_issues, through: :prometheus_alert_events
@@ -19,6 +19,7 @@ class PrometheusAlert < ApplicationRecord
   after_save :clear_prometheus_adapter_cache!
   after_destroy :clear_prometheus_adapter_cache!
 
+  validates :environment, :project, :prometheus_metric, presence: true
   validate :require_valid_environment_project!
   validate :require_valid_metric_project!
 

spec/models/prometheus_alert_spec.rb

@@ -31,8 +31,8 @@ describe PrometheusAlert do
   end
 
   describe 'associations' do
-    it { is_expected.to belong_to(:project).required }
-    it { is_expected.to belong_to(:environment).required }
+    it { is_expected.to belong_to(:project) }
+    it { is_expected.to belong_to(:environment) }
   end
 
   describe 'project validations' do
@@ -43,6 +43,10 @@ describe PrometheusAlert do
       build(:prometheus_alert, prometheus_metric: metric, environment: environment, project: project)
     end
 
+    it { is_expected.to validate_presence_of(:environment) }
+    it { is_expected.to validate_presence_of(:project) }
+    it { is_expected.to validate_presence_of(:prometheus_metric) }
+
     context 'when environment and metric belongs same project' do
       it { is_expected.to be_valid }
     end