Add less-strict index for alert fingerprints

- Adjust specs, migration, model specs

app/models/alert_management/alert.rb

@@ -55,7 +55,7 @@ module AlertManagement
     validates :severity,        presence: true
     validates :status,          presence: true
     validates :started_at,      presence: true
-    validates :fingerprint,     uniqueness: { scope: :project }, allow_blank: true
+    validates :fingerprint,     uniqueness: { scope: :project }, allow_blank: true, unless: :resolved?
     validate  :hosts_length
 
     enum severity: {

db/migrate/20200706035141_adjust_unique_index_alert_management_alerts.rb

+# frozen_string_literal: true
+
+class AdjustUniqueIndexAlertManagementAlerts < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME        = false
+  INDEX_NAME      = 'index_alert_management_alerts_on_project_id_and_fingerprint'
+  RESOLVED_STATUS = AlertManagement::Alert::STATUSES[:resolved]
+
+  # rubocop:disable Migration/RemoveIndex
+  # rubocop:disable Migration/AddIndex
+  def up
+    remove_index :alert_management_alerts, name: INDEX_NAME
+    add_index(:alert_management_alerts, %w(project_id fingerprint), where: "status <> '#{RESOLVED_STATUS}'", name: INDEX_NAME, unique: true, using: :btree)
+  end
+
+  def down
+    remove_index :alert_management_alerts, name: INDEX_NAME
+    add_index(:alert_management_alerts, %w(project_id fingerprint), name: INDEX_NAME, unique: true, using: :btree)
+  end
+  # rubocop:enable Migration/RemoveIndex
+  # rubocop:enable Migration/AddIndex
+end

spec/models/alert_management/alert_spec.rb

@@ -94,6 +94,13 @@ RSpec.describe AlertManagement::Alert do
           let(:project) { existing_alert.project }
 
           it { is_expected.not_to be_valid }
+
+          context 'resolved status' do
+            # We are only validating uniqueness for non-resolved alerts
+            let(:new_alert) { build(:alert_management_alert, :resolved, fingerprint: fingerprint, project: project) }
+
+            it { is_expected.to be_valid }
+          end
         end
 
         context 'different project' do