Change IssueSetEpic mutation epic_id argument

- Change epic_id to be not required so the value
can be nil. This allows the mutation to remove an
existing epic assigned to the issue
- Add test example to IssueSetEpic request tests
- Update schema

doc/api/graphql/reference/gitlab_schema.graphql

@@ -7355,9 +7355,9 @@ input IssueSetEpicInput {
   clientMutationId: String
 
   """
-  Global ID of the epic to be assigned to the issue
+  Global ID of the epic to be assigned to the issue, epic will be removed if absent or set to null
   """
-  epicId: ID!
+  epicId: ID
 
   """
   The IID of the issue to mutate

doc/api/graphql/reference/gitlab_schema.json

@@ -20359,15 +20359,11 @@
             },
             {
               "name": "epicId",
-              "description": "Global ID of the epic to be assigned to the issue",
+              "description": "Global ID of the epic to be assigned to the issue, epic will be removed if absent or set to null",
               "type": {
-                "kind": "NON_NULL",
-                "name": null,
-                "ofType": {
                 "kind": "SCALAR",
                 "name": "ID",
                 "ofType": null
-                }
               },
               "defaultValue": null
             },

ee/app/graphql/mutations/issues/set_epic.rb

@@ -7,9 +7,10 @@ module Mutations
 
       argument :epic_id,
                GraphQL::ID_TYPE,
-               required: true,
+               required: false,
                loads: Types::EpicType,
-               description: 'Global ID of the epic to be assigned to the issue'
+               description: 'Global ID of the epic to be assigned to the issue, ' \
+               'epic will be removed if absent or set to null'
 
       def resolve(project_path:, iid:, epic: nil)
         issue = authorized_find!(project_path: project_path, iid: iid)

ee/changelogs/unreleased/229506-graphql-mutation-for-adding-epic-to-issue-2.yml

+---
+title: Change IssueSetEpic mutation to make epic_id argument not required
+merge_request: 39451
+author:
+type: changed

ee/spec/requests/api/graphql/mutations/issues/set_epic_spec.rb

@@ -5,12 +5,12 @@ require 'spec_helper'
 RSpec.describe 'Setting the epic of an issue' do
   include GraphqlHelpers
 
-  let(:current_user) { create(:user) }
-  let(:group) { create(:group) }
-  let(:epic) { create(:epic, group: group) }
-  let(:project) { create(:project, group: group) }
-  let(:issue) { create(:issue, project: project) }
-  let(:input) { { epic_id: global_id_of(epic) } }
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:group) { create(:group) }
+  let_it_be(:epic) { create(:epic, group: group) }
+  let_it_be(:project) { create(:project, group: group) }
+  let_it_be(:issue) { create(:issue, project: project, epic: create(:epic, group: group)) }
+  let_it_be(:input) { { epic_id: global_id_of(epic) } }
 
   let(:mutation) do
     graphql_mutation(
@@ -65,4 +65,12 @@ RSpec.describe 'Setting the epic of an issue' do
     expect(mutation_response['issue']['epic']['title']).to eq(epic.title)
     expect(issue.reload.epic).to eq(epic)
   end
+
+  it 'removes existing epic if epic_id is nil' do
+    input[:epic_id] = nil
+    post_graphql_mutation(mutation, current_user: current_user)
+
+    expect(response).to have_gitlab_http_status(:success)
+    expect(issue.reload.epic).to be_nil
+  end
 end