Add singular resolver for oncall rotations

- Move from using association directly to finder
- Add specs for new resolver

doc/api/graphql/reference/index.md

@@ -3333,6 +3333,7 @@ Describes an incident management on-call schedule.
 | `description` | [`String`](#string) | Description of the on-call schedule. |
 | `iid` | [`ID!`](#id) | Internal ID of the on-call schedule. |
 | `name` | [`String!`](#string) | Name of the on-call schedule. |
+| `rotation` | [`IncidentManagementOncallRotation!`](#incidentmanagementoncallrotation) | On-call rotation for the on-call schedule. |
 | `rotations` | [`IncidentManagementOncallRotationConnection!`](#incidentmanagementoncallrotationconnection) | On-call rotations for the on-call schedule. |
 | `timezone` | [`String!`](#string) | Time zone of the on-call schedule. |
 

ee/app/graphql/resolvers/incident_management/oncall_rotations_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  module IncidentManagement
+    class OncallRotationsResolver < BaseResolver
+      alias_method :schedule, :object
+
+      type Types::IncidentManagement::OncallRotationType.connection_type, null: true
+
+      when_single do
+        argument :id,
+                 ::Types::GlobalIDType[::IncidentManagement::OncallRotation],
+                 required: true,
+                 description: 'ID of the on-call rotation.',
+                 prepare: ->(id, ctx) { id.model_id }
+      end
+
+      def resolve(**args)
+        ::IncidentManagement::OncallRotationsFinder.new(context[:current_user], schedule.project, schedule, args).execute
+      end
+    end
+  end
+end

ee/app/graphql/types/incident_management/oncall_schedule_type.rb

@@ -31,7 +31,14 @@ module Types
       field :rotations,
             OncallRotationType.connection_type,
             null: false,
-            description: 'On-call rotations for the on-call schedule.'
+            description: 'On-call rotations for the on-call schedule.',
+            resolver: ::Resolvers::IncidentManagement::OncallRotationsResolver
+
+      field :rotation,
+            OncallRotationType,
+            null: false,
+            description: 'On-call rotation for the on-call schedule.',
+            resolver: ::Resolvers::IncidentManagement::OncallRotationsResolver.single
     end
   end
 end

ee/spec/graphql/resolvers/incident_management/oncall_rotations_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::IncidentManagement::OncallRotationsResolver do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:rotation) { create(:incident_management_oncall_rotation, :with_participants, :utc) }
+  let_it_be(:second_rotation) { create(:incident_management_oncall_rotation, :with_participants, :utc, schedule: rotation.schedule) }
+  let_it_be(:schedule) { rotation.schedule }
+  let_it_be(:project) { rotation.project }
+
+  let(:args) { {} }
+  let(:resolver) { described_class }
+
+  subject(:resolved_rotations) { sync(resolve_oncall_rotations(args, current_user: current_user).to_a) }
+
+  before do
+    stub_licensed_features(oncall_schedules: true)
+    project.add_reporter(current_user)
+  end
+
+  specify do
+    expect(described_class).to have_nullable_graphql_type(Types::IncidentManagement::OncallRotationType.connection_type)
+  end
+
+  it 'returns on-call rotations' do
+    expect(resolved_rotations.length).to eq(2)
+    expect(resolved_rotations.first).to be_a(::IncidentManagement::OncallRotation)
+    expect(resolved_rotations.first).to have_attributes(id: second_rotation.id)
+    expect(resolved_rotations.last).to have_attributes(id: rotation.id)
+  end
+
+  context 'when user does not have permissions' do
+    let(:another_user) { create(:user) }
+
+    subject(:resolved_rotations) { sync(resolve_oncall_rotations(args, current_user: another_user).to_a) }
+
+    it 'returns no rotations' do
+      expect(resolved_rotations.length).to eq(0)
+    end
+  end
+
+  context 'when resolving a single item' do
+    let(:resolver) { described_class.single }
+
+    subject(:resolved_rotation) { sync(resolve_oncall_rotations(args, current_user: current_user)) }
+
+    context 'when id given' do
+      let(:args) { { id: rotation.to_global_id } }
+
+      it 'returns the on-call rotation' do
+        expect(resolved_rotation).to eq(rotation)
+      end
+    end
+  end
+
+  private
+
+  def resolve_oncall_rotations(args = {}, context = { current_user: current_user })
+    resolve(resolver, obj: schedule, args: args, ctx: context)
+  end
+end

ee/spec/graphql/types/incident_management/oncall_schedule_type_spec.rb

@@ -14,6 +14,7 @@ RSpec.describe GitlabSchema.types['IncidentManagementOncallSchedule'] do
       description
       timezone
       rotations
+      rotation
     ]
 
     expect(described_class).to have_graphql_fields(*expected_fields)