Add GraphQL mutation for adding oncall rotation

- Add oncall rotation type + enums
- Add mutation
- Update docs

doc/api/graphql/reference/gitlab_schema.graphql

@@ -10870,6 +10870,66 @@ An ISO 8601-encoded date
 """
 scalar ISO8601Date
 
+"""
+Describes an incident management on-call rotation
+"""
+type IncidentManagementOncallRotation {
+  """
+  ID of the on-call rotation
+  """
+  id: IncidentManagementOncallRotationID!
+
+  """
+  Time zone of the on-call schedule
+  """
+  length: Int
+
+  """
+  Unit of the on-call rotation length
+  """
+  lengthUnit: OncallRotationUnitEnum
+
+  """
+  Name of the on-call rotation
+  """
+  name: String!
+
+  """
+  Participants of the on-call rotation
+  """
+  participants(
+    """
+    Returns the elements in the list that come after the specified cursor.
+    """
+    after: String
+
+    """
+    Returns the elements in the list that come before the specified cursor.
+    """
+    before: String
+
+    """
+    Returns the first _n_ elements from the list.
+    """
+    first: Int
+
+    """
+    Returns the last _n_ elements from the list.
+    """
+    last: Int
+  ): OncallUserTypeConnection
+
+  """
+  Start date of the on-call rotation
+  """
+  startsAt: Time
+}
+
+"""
+Identifier of IncidentManagement::OncallRotation
+"""
+scalar IncidentManagementOncallRotationID
+
 """
 Describes an incident management on-call schedule
 """
@@ -14794,6 +14854,7 @@ type Mutation {
   """
   mergeRequestUpdate(input: MergeRequestUpdateInput!): MergeRequestUpdatePayload
   namespaceIncreaseStorageTemporarily(input: NamespaceIncreaseStorageTemporarilyInput!): NamespaceIncreaseStorageTemporarilyPayload
+  oncallRotationCreate(input: OncallRotationCreateInput!): OncallRotationCreatePayload
   oncallScheduleCreate(input: OncallScheduleCreateInput!): OncallScheduleCreatePayload
   oncallScheduleDestroy(input: OncallScheduleDestroyInput!): OncallScheduleDestroyPayload
   oncallScheduleUpdate(input: OncallScheduleUpdateInput!): OncallScheduleUpdatePayload
@@ -15404,6 +15465,116 @@ Identifier of Noteable
 """
 scalar NoteableID
 
+"""
+Autogenerated input type of OncallRotationCreate
+"""
+input OncallRotationCreateInput {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+
+  """
+  The name of the on-call rotation
+  """
+  name: String!
+
+  """
+  The usernames of users participating in the on-call rotation
+  """
+  participants: [OncallUserInputType!]!
+
+  """
+  The project to create the on-call schedule in
+  """
+  projectPath: ID!
+
+  """
+  The rotation length of the on-call rotation
+  """
+  rotationLength: OncallRotationLengthInputType!
+
+  """
+  The iid of the on-call schedule to create the on-call rotation in
+  """
+  scheduleIid: String!
+
+  """
+  The start date and time of the on-call rotation
+  """
+  startsAt: OncallRotationDateInputType!
+}
+
+"""
+Autogenerated return type of OncallRotationCreate
+"""
+type OncallRotationCreatePayload {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+
+  """
+  Errors encountered during execution of the mutation.
+  """
+  errors: [String!]!
+
+  """
+  The on-call rotation
+  """
+  oncallRotation: IncidentManagementOncallRotation
+}
+
+"""
+Date input type for on-call rotation
+"""
+input OncallRotationDateInputType {
+  """
+  The date component of the date in YYYY-MM-DD format
+  """
+  date: String!
+
+  """
+  The time component of the date in 24hr HH:MM format
+  """
+  time: String!
+}
+
+"""
+The rotation length of the on-call rotation
+"""
+input OncallRotationLengthInputType {
+  """
+  The rotation length of the on-call rotation
+  """
+  length: Int!
+
+  """
+  The unit of the rotation length of the on-call rotation
+  """
+  unit: OncallRotationUnitEnum!
+}
+
+"""
+Rotation length unit of an on-call rotation
+"""
+enum OncallRotationUnitEnum {
+  """
+  One Days
+  """
+  DAYS
+
+  """
+  One Hours
+  """
+  HOURS
+
+  """
+  One Weeks
+  """
+  WEEKS
+}
+
 """
 Autogenerated input type of OncallScheduleCreate
 """
@@ -15549,6 +15720,81 @@ type OncallScheduleUpdatePayload {
   oncallSchedule: IncidentManagementOncallSchedule
 }
 
+"""
+The rotation user and color palette
+"""
+input OncallUserInputType {
+  """
+  The color palette to assign to the on-call user, for example: "blue".
+  """
+  colorPalette: String
+
+  """
+  The color weight to assign to for the on-call user, for example "500". Max 4 chars. For easy identification of the user.
+  """
+  colorWeight: String
+
+  """
+  The username of the user to participate in the on-call rotation. i.e user_one
+  """
+  username: String!
+}
+
+"""
+The rotation user and color palette
+"""
+type OncallUserType {
+  """
+  The color palette to assign to the on-call user. For example "blue".
+  """
+  colorPalette: String
+
+  """
+  The color weight to assign to for the on-call user, for example "500". Max 4 chars. For easy identification of the user.
+  """
+  colorWeight: String
+
+  """
+  The user who is participating
+  """
+  user: User!
+}
+
+"""
+The connection type for OncallUserType.
+"""
+type OncallUserTypeConnection {
+  """
+  A list of edges.
+  """
+  edges: [OncallUserTypeEdge]
+
+  """
+  A list of nodes.
+  """
+  nodes: [OncallUserType]
+
+  """
+  Information to aid in pagination.
+  """
+  pageInfo: PageInfo!
+}
+
+"""
+An edge in a connection.
+"""
+type OncallUserTypeEdge {
+  """
+  A cursor for use in pagination.
+  """
+  cursor: String!
+
+  """
+  The item at the end of the edge.
+  """
+  node: OncallUserType
+}
+
 """
 Represents a package
 """

doc/api/graphql/reference/index.md

@@ -1712,6 +1712,19 @@ Autogenerated return type of HttpIntegrationUpdate.
 | `errors` | String! => Array | Errors encountered during execution of the mutation. |
 | `integration` | AlertManagementHttpIntegration | The HTTP integration |
 
+### IncidentManagementOncallRotation
+
+Describes an incident management on-call rotation.
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `id` | IncidentManagementOncallRotationID! | ID of the on-call rotation |
+| `length` | Int | Time zone of the on-call schedule |
+| `lengthUnit` | OncallRotationUnitEnum | Unit of the on-call rotation length |
+| `name` | String! | Name of the on-call rotation |
+| `participants` | OncallUserTypeConnection | Participants of the on-call rotation |
+| `startsAt` | Time | Start date of the on-call rotation |
+
 ### IncidentManagementOncallSchedule
 
 Describes an incident management on-call schedule.
@@ -2377,6 +2390,16 @@ Autogenerated return type of NamespaceIncreaseStorageTemporarily.
 | `repositionNote` | Boolean! | Indicates the user can perform `reposition_note` on this resource |
 | `resolveNote` | Boolean! | Indicates the user can perform `resolve_note` on this resource |
 
+### OncallRotationCreatePayload
+
+Autogenerated return type of OncallRotationCreate.
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `clientMutationId` | String | A unique identifier for the client performing the mutation. |
+| `errors` | String! => Array | Errors encountered during execution of the mutation. |
+| `oncallRotation` | IncidentManagementOncallRotation | The on-call rotation |
+
 ### OncallScheduleCreatePayload
 
 Autogenerated return type of OncallScheduleCreate.
@@ -2407,6 +2430,16 @@ Autogenerated return type of OncallScheduleUpdate.
 | `errors` | String! => Array | Errors encountered during execution of the mutation. |
 | `oncallSchedule` | IncidentManagementOncallSchedule | The on-call schedule |
 
+### OncallUserType
+
+The rotation user and color palette.
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `colorPalette` | String | The color palette to assign to the on-call user. For example "blue". |
+| `colorWeight` | String | The color weight to assign to for the on-call user, for example "500". Max 4 chars. For easy identification of the user. |
+| `user` | User! | The user who is participating |
+
 ### Package
 
 Represents a package.
@@ -4574,6 +4607,16 @@ Values for sorting projects.
 | `SIMILARITY` | Most similar to the search query |
 | `STORAGE` | Sort by storage size |
 
+### OncallRotationUnitEnum
+
+Rotation length unit of an on-call rotation.
+
+| Value | Description |
+| ----- | ----------- |
+| `DAYS` | One Days |
+| `HOURS` | One Hours |
+| `WEEKS` | One Weeks |
+
 ### PackageTypeEnum
 
 | Value | Description |

ee/app/graphql/ee/types/mutation_type.rb

@@ -55,6 +55,7 @@ module EE
         mount_mutation ::Mutations::IncidentManagement::OncallSchedule::Create
         mount_mutation ::Mutations::IncidentManagement::OncallSchedule::Update
         mount_mutation ::Mutations::IncidentManagement::OncallSchedule::Destroy
+        mount_mutation ::Mutations::IncidentManagement::OncallRotation::Create
 
         prepend(Types::DeprecatedMutations)
       end

ee/app/graphql/mutations/incident_management/oncall_rotation/base.rb

+# frozen_string_literal: true
+
+module Mutations
+  module IncidentManagement
+    module OncallRotation
+      class Base < BaseMutation
+        field :oncall_rotation,
+              ::Types::IncidentManagement::OncallRotationType,
+              null: true,
+              description: 'The on-call rotation'
+
+        authorize :admin_incident_management_oncall_schedule
+
+        private
+
+        def response(result, errors = nil)
+          {
+            oncall_rotation: result.payload[:oncall_rotation],
+            errors: errors.presence || result.errors
+          }
+        end
+      end
+    end
+  end
+end

ee/app/graphql/mutations/incident_management/oncall_rotation/create.rb

+# frozen_string_literal: true
+
+module Mutations
+  module IncidentManagement
+    module OncallRotation
+      class Create < Base
+        include ResolvesProject
+
+        graphql_name 'OncallRotationCreate'
+
+        argument :project_path, GraphQL::ID_TYPE,
+                 required: true,
+                 description: 'The project to create the on-call schedule in'
+
+        argument :schedule_iid, GraphQL::STRING_TYPE,
+                 required: true,
+                 description: 'The iid of the on-call schedule to create the on-call rotation in',
+                 as: :iid
+
+        argument :name, GraphQL::STRING_TYPE,
+                 required: true,
+                 description: 'The name of the on-call rotation'
+
+        argument :starts_at, Types::IncidentManagement::OncallRotationDateInputType,
+                 required: true,
+                 description: 'The start date and time of the on-call rotation'
+
+        argument :rotation_length, Types::IncidentManagement::OncallRotationLengthInputType,
+                 required: true,
+                 description: 'The rotation length of the on-call rotation'
+
+        argument :participants,
+                 [Types::IncidentManagement::OncallUserInputType],
+                 required: true,
+                 description: 'The usernames of users participating in the on-call rotation'
+
+        MAXIMUM_PARTICIPANTS = 100
+
+        def resolve(iid:, project_path:, participants:, **args)
+          project = authorized_find!(full_path: project_path)
+
+          schedule = ::IncidentManagement::OncallSchedulesFinder.new(current_user, project, iids: iid)
+                                                                .execute
+                                                                .first
+
+          raise_schedule_not_found unless schedule
+
+          result = ::IncidentManagement::OncallRotations::CreateService.new(
+            schedule,
+            project,
+            current_user,
+            prepare_params(schedule, participants, args)
+          ).execute
+
+          errors = result.error? ? [result.message] : []
+
+          response(result, errors)
+        end
+
+        private
+
+        def prepare_params(schedule, participants, args)
+          rotation_length = args[:rotation_length][:length]
+          rotation_length_unit = args[:rotation_length][:unit]
+          starts_at = parse_start_time(schedule, args)
+
+          args.slice(:name).merge(
+            length: rotation_length,
+            length_unit: rotation_length_unit,
+            starts_at: starts_at,
+            participants: find_participants(participants)
+          )
+        end
+
+        def parse_start_time(schedule, args)
+          args[:starts_at].asctime.in_time_zone(schedule.timezone)
+        end
+
+        def find_object(full_path:)
+          resolve_project(full_path: full_path)
+        end
+
+        def find_participants(user_array)
+          raise_too_many_users_error if user_array.size > MAXIMUM_PARTICIPANTS
+
+          usernames = user_array.map {|h| h[:username] }
+          matched_users = UsersFinder.new(current_user, username: usernames).execute.order_by(:username)
+
+          raise_user_not_found if matched_users.size != user_array.size
+
+          user_array = user_array.sort_by! { |h| h[:username] }
+
+          user_array.map.with_index { |param, i| param.to_h.merge(user: matched_users[i]) }
+        end
+
+        def raise_schedule_not_found
+          raise Gitlab::Graphql::Errors::ArgumentError, 'The schedule could not be found'
+        end
+
+        def raise_too_many_users_error
+          raise Gitlab::Graphql::Errors::ArgumentError, "A maximum of #{MAXIMUM_PARTICIPANTS} participants can be added"
+        end
+
+        def raise_user_not_found
+          raise Gitlab::Graphql::Errors::ArgumentError, 'A username that was provided could not be matched to a user'
+        end
+      end
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    # rubocop: disable Graphql/AuthorizeTypes
+    class OncallRotationDateInputType < BaseInputObject
+      graphql_name 'OncallRotationDateInputType'
+      description 'Date input type for on-call rotation'
+
+      argument :date, GraphQL::STRING_TYPE,
+                required: true,
+                description: 'The date component of the date in YYYY-MM-DD format'
+
+      argument :time, GraphQL::STRING_TYPE,
+                required: true,
+                description: 'The time component of the date in 24hr HH:MM format'
+
+      DATE_FORMAT = %r[\d{4}-[0123]\d-\d{2}].freeze
+      TIME_FORMAT = %r[[012]\d:\d{2}].freeze
+
+      def prepare
+        raise Gitlab::Graphql::Errors::ArgumentError, 'Date given is invalid' unless DATE_FORMAT.match(date)
+        raise Gitlab::Graphql::Errors::ArgumentError, 'Time given is invalid' unless TIME_FORMAT.match(time)
+
+        Time.parse("#{date} #{time}")
+      rescue ArgumentError, TypeError
+        raise Gitlab::Graphql::Errors::ArgumentError, 'Date & time is invalid'
+      end
+    end
+    # rubocop: enable Graphql/AuthorizeTypes
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    # rubocop: disable Graphql/AuthorizeTypes
+    class OncallRotationLengthInputType < BaseInputObject
+      graphql_name 'OncallRotationLengthInputType'
+      description 'The rotation length of the on-call rotation'
+
+      argument :length, GraphQL::INT_TYPE,
+                required: true,
+                description: 'The rotation length of the on-call rotation'
+
+      argument :unit, Types::IncidentManagement::OncallRotationLengthUnitEnum,
+                required: true,
+                description: 'The unit of the rotation length of the on-call rotation'
+    end
+    # rubocop: enable Graphql/AuthorizeTypes
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    class OncallRotationLengthUnitEnum < BaseEnum
+      graphql_name 'OncallRotationUnitEnum'
+      description 'Rotation length unit of an on-call rotation'
+
+      ::IncidentManagement::OncallRotation.length_units.keys.each do |unit|
+        value unit.upcase, value: unit, description: "One #{unit.titleize}"
+      end
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    class OncallRotationType < BaseObject
+      graphql_name 'IncidentManagementOncallRotation'
+      description 'Describes an incident management on-call rotation'
+
+      authorize :read_incident_management_oncall_schedule
+
+      field :id,
+            Types::GlobalIDType[::IncidentManagement::OncallRotation],
+            null: false,
+            description: 'ID of the on-call rotation'
+
+      field :name,
+            GraphQL::STRING_TYPE,
+            null: false,
+            description: 'Name of the on-call rotation'
+
+      field :starts_at,
+            Types::TimeType,
+            null: true,
+            description: 'Start date of the on-call rotation'
+
+      field :length,
+            GraphQL::INT_TYPE,
+            null: true,
+            description: 'Time zone of the on-call schedule'
+
+      field :length_unit,
+            Types::IncidentManagement::OncallRotationLengthUnitEnum,
+            null: true,
+            description: 'Unit of the on-call rotation length'
+
+      field :participants,
+            ::Types::IncidentManagement::OncallUserType.connection_type,
+            null: true,
+            description: 'Participants of the on-call rotation'
+
+      def participants
+        object.oncall_participants
+      end
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    # rubocop: disable Graphql/AuthorizeTypes
+    class OncallUserInputType < BaseInputObject
+      graphql_name 'OncallUserInputType'
+      description 'The rotation user and color palette'
+
+      argument :username, GraphQL::STRING_TYPE,
+                required: true,
+                description: 'The username of the user to participate in the on-call rotation. i.e user_one'
+
+      argument :color_palette, GraphQL::STRING_TYPE,
+                required: false,
+                description: 'The color palette to assign to the on-call user, for example: "blue".'
+
+      argument :color_weight, GraphQL::STRING_TYPE,
+                required: false,
+                description: 'The color weight to assign to for the on-call user, for example "500". Max 4 chars. For easy identification of the user.'
+    end
+    # rubocop: enable Graphql/AuthorizeTypes
+  end
+end

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

+# frozen_string_literal: true
+
+module Types
+  module IncidentManagement
+    # rubocop: disable Graphql/AuthorizeTypes
+    class OncallUserType < BaseObject
+      graphql_name 'OncallUserType'
+      description 'The rotation user and color palette'
+
+      field :user, Types::UserType,
+                null: false,
+                description: 'The user who is participating'
+
+      field :color_palette, GraphQL::STRING_TYPE,
+                null: true,
+                description: 'The color palette to assign to the on-call user. For example "blue".'
+
+      field :color_weight, GraphQL::STRING_TYPE,
+                null: true,
+                description: 'The color weight to assign to for the on-call user, for example "500". Max 4 chars. For easy identification of the user.'
+    end
+    # rubocop: enable Graphql/AuthorizeTypes
+  end
+end

ee/spec/graphql/mutations/incident_management/oncall_rotation/create_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Mutations::IncidentManagement::OncallRotation::Create do
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:schedule) { create(:incident_management_oncall_schedule, project: project) }
+  let(:args) do
+    {
+      project_path: project.full_path,
+      name: 'On-call rotation',
+      schedule_iid: schedule.iid,
+      starts_at: "2020-01-10 09:00".in_time_zone(schedule.timezone),
+      rotation_length: {
+        length: 1,
+        unit: ::IncidentManagement::OncallRotation.rotation_length_units[:days]
+      },
+      participants: [
+        {
+          username: current_user.username,
+          color_weight: "500",
+          color_palette: "black"
+        }
+      ]
+    }
+  end
+
+  specify { expect(described_class).to require_graphql_authorizations(:admin_incident_management_oncall_schedule) }
+
+  describe '#resolve' do
+    subject(:resolve) { mutation_for(project, current_user).resolve(iid: schedule.iid, project_path: project.full_path, participants: args[:participants], **args) }
+
+    context 'user has access to project' do
+      before do
+        stub_licensed_features(oncall_schedules: true)
+        project.add_maintainer(current_user)
+      end
+
+      context 'when OncallRotation::CreateService responds with success' do
+        it 'returns the on-call rotation with no errors' do
+          expect(resolve).to match(
+            oncall_rotation: ::IncidentManagement::OncallRotation.last!,
+            errors: be_empty
+          )
+        end
+      end
+
+      context 'when OncallRotations::CreateService responds with an error' do
+        before do
+          allow_any_instance_of(::IncidentManagement::OncallRotations::CreateService)
+            .to receive(:execute)
+            .and_return(ServiceResponse.error(payload: { oncall_rotation: nil }, message: 'An on-call rotation already exists'))
+        end
+
+        it 'returns errors' do
+          expect(resolve).to eq(
+            oncall_rotation: nil,
+            errors: ['An on-call rotation already exists']
+          )
+        end
+      end
+
+      describe 'error cases' do
+        context 'user cannot be found' do
+          before do
+            args.merge!(participants: [username: 'unknown'])
+          end
+
+          it 'raises an error' do
+            expect { resolve }.to raise_error(Gitlab::Graphql::Errors::ArgumentError, 'A username that was provided could not be matched to a user')
+          end
+        end
+
+        context 'user does not have access to the project' do
+          before do
+            other_user = create(:user)
+            args.merge!(
+              participants: [
+                {
+                  username: other_user.username,
+                  color_weight: "500",
+                  color_palette: "black"
+                }
+              ]
+            )
+          end
+
+          it 'raises an error' do
+            expect { resolve }.to raise_error(ActiveRecord::RecordInvalid, /participant does not have access to the project/)
+          end
+        end
+
+        context 'schedule does not exist' do
+          let(:schedule) { double(iid: non_existing_record_iid, timezone: 'UTC') }
+
+          it 'raises an error' do
+            expect { resolve }.to raise_error(Gitlab::Graphql::Errors::ArgumentError, 'The schedule could not be found')
+          end
+        end
+
+        context 'too many users' do
+          before do
+            stub_const('Mutations::IncidentManagement::OncallRotation::Create::MAXIMUM_PARTICIPANTS', 0)
+          end
+
+          it 'raises an error' do
+            expect { resolve }.to raise_error(Gitlab::Graphql::Errors::ArgumentError, "A maximum of #{described_class::MAXIMUM_PARTICIPANTS} participants can be added")
+          end
+        end
+      end
+    end
+
+    context 'when resource is not accessible to the user' do
+      it 'raises an error' do
+        expect { resolve }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
+      end
+    end
+  end
+
+  private
+
+  def mutation_for(project, user)
+    described_class.new(object: project, context: { current_user: user }, field: nil)
+  end
+end

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe GitlabSchema.types['IncidentManagementOncallRotation'] do
+  specify { expect(described_class.graphql_name).to eq('IncidentManagementOncallRotation') }
+
+  specify { expect(described_class).to require_graphql_authorizations(:read_incident_management_oncall_schedule) }
+
+  it 'exposes the expected fields' do
+    expected_fields = %i[
+      id
+      name
+      starts_at
+      rotation_length
+      rotation_length_unit
+      participants
+    ]
+
+    expect(described_class).to have_graphql_fields(*expected_fields)
+  end
+end

ee/spec/requests/api/graphql/mutations/incident_management/oncall_rotation/create_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Creating a new on-call schedule' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:schedule) { create(:incident_management_oncall_schedule, project: project) }
+  let(:args) do
+    {
+      project_path: project.full_path,
+      name: 'On-call rotation',
+      schedule_iid: schedule.iid.to_s,
+      starts_at: {
+        date: "2020-09-19",
+        time: "09:00"
+      },
+      rotation_length: {
+        length: 1,
+        unit: 'DAYS'
+      },
+      participants: [
+        {
+          username: current_user.username,
+          colorWeight: "500",
+          colorPalette: "black"
+        }
+      ]
+    }
+  end
+
+  let(:mutation) do
+    graphql_mutation(:oncall_rotation_create, args) do
+      <<~QL
+        clientMutationId
+        errors
+        oncallRotation {
+          id
+          name
+          startsAt
+          length
+          lengthUnit
+          participants {
+            nodes {
+              user {
+                id
+                username
+              }
+            }
+          }
+        }
+      QL
+    end
+  end
+
+  let(:mutation_response) { graphql_mutation_response(:oncall_rotation_create) }
+
+  before do
+    stub_licensed_features(oncall_schedules: true)
+    project.add_maintainer(current_user)
+  end
+
+  it 'creates a new on-call rotation', :aggregate_failures do
+    post_graphql_mutation(mutation, current_user: current_user)
+
+    new_oncall_rotation = ::IncidentManagement::OncallRotation.last!
+    oncall_rotation_response = mutation_response['oncallRotation']
+
+    expect(response).to have_gitlab_http_status(:success)
+
+    expect(oncall_rotation_response.slice(*%w[id name length lengthUnit])).to eq(
+      'id' => global_id_of(new_oncall_rotation),
+      'name' => args[:name],
+      'length' => 1,
+      'lengthUnit' => 'DAYS'
+    )
+
+    start_time = "#{args[:starts_at][:date]} #{args[:starts_at][:time]}".in_time_zone(schedule.timezone)
+    expect(Time.parse(oncall_rotation_response['startsAt'])).to eq(start_time)
+
+    expect(oncall_rotation_response.dig('participants', 'nodes')).to contain_exactly(
+      {
+        'user' => {
+          'id' => global_id_of(current_user),
+          'username' => current_user.username
+        }
+      }
+    )
+  end
+
+  %i[project_path schedule_iid name starts_at rotation_length participants].each do |argument|
+    context "without required argument #{argument}" do
+      before do
+        args.delete(argument)
+      end
+
+      it_behaves_like 'an invalid argument to the mutation', argument_name: argument
+    end
+  end
+
+  context 'time is invalid' do
+    before do
+      args[:starts_at][:time] = '999:999'
+    end
+
+    it 'returns the on-call rotation with errors' do
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      expect(graphql_errors).not_to be_empty
+      error = graphql_errors.first.dig('extensions', 'problems', 0, 'explanation')
+      expect(error).to match('Time given is invalid')
+    end
+  end
+
+  context 'date is invalid' do
+    before do
+      args[:starts_at][:date] = 'incorrect date'
+    end
+
+    it 'returns the on-call rotation with errors' do
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      expect(graphql_errors).not_to be_empty
+      error = graphql_errors.first.dig('extensions', 'problems', 0, 'explanation')
+      expect(error).to match('Date given is invalid')
+    end
+  end
+end