Allow passing in an epic parent_id when updating epics

Changelog: added
EE: true

doc/api/epics.md

@@ -366,21 +366,22 @@ PUT /groups/:id/epics/:epic_iid
 | ------------------- | ---------------- | ---------- | ---------------------------------------------------------------------------------------|
 | `id`                | integer/string   | yes        | The ID or [URL-encoded path of the group](index.md#namespaced-path-encoding) owned by the authenticated user                |
 | `epic_iid`          | integer/string   | yes        | The internal ID of the epic  |
-| `title`             | string           | no         | The title of an epic |
-| `description`       | string           | no         | The description of an epic. Limited to 1,048,576 characters.  |
+| `add_labels`        | string           | no         | Comma-separated label names to add to an issue. |
 | `confidential`      | boolean          | no         | Whether the epic should be confidential |
+| `description`       | string           | no         | The description of an epic. Limited to 1,048,576 characters.  |
+| `due_date_fixed`    | string           | no         | The fixed due date of an epic (in GitLab 11.3 and later) |
+| `due_date_is_fixed` | boolean          | no         | Whether due date should be sourced from `due_date_fixed` or from milestones (in GitLab 11.3 and later) |
 | `labels`            | string           | no         | Comma-separated label names for an issue. Set to an empty string to unassign all labels. |
-| `add_labels`        | string           | no         | Comma-separated label names to add to an issue. |
+| `parent_id`         | integer/string   | no         | The ID of a parent epic. Available in [GitLab 14.6](https://gitlab.com/gitlab-org/gitlab/-/issues/348123) and later |
 | `remove_labels`     | string           | no         | Comma-separated label names to remove from an issue. |
-| `updated_at`        | string           | no         | When the epic was updated. Date time string, ISO 8601 formatted, for example `2016-03-11T03:45:40Z` . Requires administrator or project/group owner privileges ([available](https://gitlab.com/gitlab-org/gitlab/-/issues/255309) in GitLab 13.5 and later) |
-| `start_date_is_fixed` | boolean        | no         | Whether start date should be sourced from `start_date_fixed` or from milestones (in GitLab 11.3 and later) |
 | `start_date_fixed`  | string           | no         | The fixed start date of an epic (in GitLab 11.3 and later) |
-| `due_date_is_fixed` | boolean          | no         | Whether due date should be sourced from `due_date_fixed` or from milestones (in GitLab 11.3 and later) |
-| `due_date_fixed`    | string           | no         | The fixed due date of an epic (in GitLab 11.3 and later) |
+| `start_date_is_fixed` | boolean        | no         | Whether start date should be sourced from `start_date_fixed` or from milestones (in GitLab 11.3 and later) |
 | `state_event`       | string           | no         | State event for an epic. Set `close` to close the epic and `reopen` to reopen it (in GitLab 11.4 and later) |
+| `title`             | string           | no         | The title of an epic |
+| `updated_at`        | string           | no         | When the epic was updated. Date time string, ISO 8601 formatted, for example `2016-03-11T03:45:40Z` . Requires administrator or project/group owner privileges ([available](https://gitlab.com/gitlab-org/gitlab/-/issues/255309) in GitLab 13.5 and later) |
 
 ```shell
-curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/1/epics/5?title=New%20Title"
+curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/1/epics/5?title=New%20Title&parent_id=29"
 ```
 
 Example response:
@@ -390,8 +391,8 @@ Example response:
   "id": 33,
   "iid": 6,
   "group_id": 7,
-  "parent_id": null,
-  "parent_iid": null,
+  "parent_id": 29,
+  "parent_iid": 4,
   "title": "New Title",
   "description": "Epic description",
   "state": "opened",

ee/lib/api/epics.rb

@@ -124,7 +124,8 @@ module API
         optional :add_labels, type: Array[String], coerce_with: Validations::Types::CommaSeparatedToArray.coerce, desc: 'Comma-separated list of label names'
         optional :remove_labels, type: Array[String], coerce_with: Validations::Types::CommaSeparatedToArray.coerce, desc: 'Comma-separated list of label names'
         optional :state_event, type: String, values: %w[reopen close], desc: 'State event for an epic'
-        at_least_one_of :title, :description, :start_date_fixed, :start_date_is_fixed, :due_date_fixed, :due_date_is_fixed, :labels, :add_labels, :remove_labels, :state_event, :confidential
+        optional :parent_id, type: Integer, desc: 'The id of a parent epic'
+        at_least_one_of :title, :description, :start_date_fixed, :start_date_is_fixed, :due_date_fixed, :due_date_is_fixed, :labels, :add_labels, :remove_labels, :state_event, :confidential, :parent_id
       end
       put ':id/(-/)epics/:epic_iid' do
         authorize_can_admin_epic!

ee/spec/requests/api/epics_spec.rb

@@ -767,6 +767,7 @@ RSpec.describe API::Epics do
 
   describe 'PUT /groups/:id/epics/:epic_iid' do
     let(:url) { "/groups/#{group.path}/epics/#{epic.iid}" }
+    let!(:epic2) { create(:epic) }
     let(:params) do
       {
         title: 'new title',
@@ -774,7 +775,8 @@ RSpec.describe API::Epics do
         labels: 'label2',
         start_date_fixed: "2018-07-17",
         start_date_is_fixed: true,
-        confidential: true
+        confidential: true,
+        parent_id: epic2.id
       }
     end
 
@@ -833,6 +835,7 @@ RSpec.describe API::Epics do
             expect(result.due_date_fixed).to eq(nil)
             expect(result.due_date_is_fixed).to be_falsey
             expect(result.confidential).to be_truthy
+            expect(result.parent_id).to eq(epic2.id)
           end
         end