Add API spec for create alert issue mutation

spec/requests/api/graphql/mutations/alert_management/alerts/create_alert_issue_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Create an alert issue from an alert' do
+  include GraphqlHelpers
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:alert) { create(:alert_management_alert, project: project) }
+
+  let(:mutation) do
+    variables = {
+      project_path: project.full_path,
+      iid: alert.iid.to_s
+    }
+    graphql_mutation(:create_alert_issue, variables,
+                     <<~QL
+                       clientMutationId
+                       errors
+                       alert {
+                         iid
+                         issueIid
+                       }
+                       issue {
+                         iid
+                         title
+                       }
+                     QL
+    )
+  end
+
+  let(:mutation_response) { graphql_mutation_response(:create_alert_issue) }
+
+  before do
+    project.add_developer(user)
+  end
+
+  context 'when there is no issue associated with the alert' do
+    it 'creates an alert issue' do
+      post_graphql_mutation(mutation, current_user: user)
+
+      new_issue = Issue.last!
+
+      expect(response).to have_gitlab_http_status(:success)
+      expect(mutation_response.slice('alert', 'issue')).to eq(
+        'alert' => {
+          'iid' => alert.iid.to_s,
+          'issueIid' => new_issue.iid.to_s
+        },
+        'issue' => {
+          'iid' => new_issue.iid.to_s,
+          'title' => new_issue.title
+        }
+      )
+    end
+  end
+
+  context 'when there is an issue already associated with the alert' do
+    before do
+      AlertManagement::CreateAlertIssueService.new(alert, user).execute
+    end
+
+    it 'responds with an error' do
+      post_graphql_mutation(mutation, current_user: user)
+
+      expect(response).to have_gitlab_http_status(:success)
+      expect(mutation_response.slice('errors', 'issue')).to eq(
+        'errors' => ['An issue already exists'],
+        'issue' => nil
+      )
+    end
+  end
+end