Graphql mutation for (un)subscribing to an epic

- incldues extraction of reused code of Epic mutations

app/models/concerns/subscribable.rb

@@ -59,6 +59,14 @@ module Subscribable
       .update(subscribed: false)
   end
 
+  def set_subscription(user, desired_state, project = nil)
+    if desired_state
+      subscribe(user, project)
+    else
+      unsubscribe(user, project)
+    end
+  end
+
   private
 
   def unsubscribe_from_other_levels(user, project)

changelogs/unreleased/34132-graphql-epic-subscribtions.yml

+---
+title: Graphql mutation for (un)subscribing to an epic
+merge_request: 19083
+author:
+type: added

doc/api/graphql/reference/index.md

@@ -271,6 +271,14 @@ The API can be explored interactively using the [GraphiQL IDE](../index.md#graph
 | `createNote` | Boolean! | Whether or not a user can perform `create_note` on this resource |
 | `awardEmoji` | Boolean! | Whether or not a user can perform `award_emoji` on this resource |
 
+### EpicSetSubscriptionPayload
+
+| Name  | Type  | Description |
+| ---   |  ---- | ----------  |
+| `clientMutationId` | String | A unique identifier for the client performing the mutation. |
+| `errors` | String! => Array | Reasons why the mutation failed. |
+| `epic` | Epic | The epic after mutation |
+
 ### EpicTreeReorderPayload
 
 | Name  | Type  | Description |

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

@@ -11,6 +11,7 @@ module EE
         mount_mutation ::Mutations::EpicTree::Reorder
         mount_mutation ::Mutations::Epics::Update
         mount_mutation ::Mutations::Epics::Create
+        mount_mutation ::Mutations::Epics::SetSubscription
       end
     end
   end

ee/app/graphql/mutations/epics/base.rb

+# frozen_string_literal: true
+
+module Mutations
+  module Epics
+    class Base < ::Mutations::BaseMutation
+      include Mutations::ResolvesGroup
+
+      private
+
+      def find_object(group_path:, iid:)
+        group = resolve_group(full_path: group_path)
+        resolver = Resolvers::EpicResolver
+                     .single.new(object: group, context: context)
+
+        resolver.resolve(iid: iid)
+      end
+    end
+  end
+end

ee/app/graphql/mutations/epics/set_subscription.rb

+# frozen_string_literal: true
+
+module Mutations
+  module Epics
+    class SetSubscription < Base
+      graphql_name 'EpicSetSubscription'
+
+      authorize :read_epic
+
+      argument :group_path, GraphQL::ID_TYPE,
+               required: true,
+               description: 'The group the epic to (un)subscribe is in'
+
+      argument :iid, GraphQL::ID_TYPE,
+               required: true,
+               description: 'The iid of the epic to (un)subscribe'
+
+      argument :subscribed_state, GraphQL::BOOLEAN_TYPE,
+               required: true,
+               description: 'The desired state of the subscription'
+
+      field :epic,
+            Types::EpicType,
+            null: true,
+            description: 'The epic after mutation'
+
+      def resolve(args)
+        group_path = args.delete(:group_path)
+        epic_iid = args.delete(:iid)
+        desired_state = args.delete(:subscribed_state)
+
+        epic = authorized_find!(group_path: group_path, iid: epic_iid)
+        epic.set_subscription(current_user, desired_state)
+
+        {
+          epic: epic.reset,
+          errors: errors_on_object(epic)
+        }
+      end
+    end
+  end
+end

ee/app/graphql/mutations/epics/update.rb

@@ -2,8 +2,7 @@
 
 module Mutations
   module Epics
-    class Update < BaseMutation
-      include Mutations::ResolvesGroup
+    class Update < Base
       prepend Mutations::SharedEpicArguments
 
       graphql_name 'UpdateEpic'
@@ -38,16 +37,6 @@ module Mutations
           errors: errors_on_object(epic)
         }
       end
-
-      private
-
-      def find_object(group_path:, iid:)
-        group = resolve_group(full_path: group_path)
-        resolver = Resolvers::EpicResolver
-          .single.new(object: group, context: context)
-
-        resolver.resolve(iid: iid)
-      end
     end
   end
 end

ee/spec/requests/api/graphql/mutations/epics/set_subscription_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Set an Epic Subscription' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:group) { create(:group) }
+  let(:epic) { create(:epic, group: group) }
+  let(:subscribed_state) { true }
+
+  let(:mutation) do
+    params = { group_path: group.full_path, iid: epic.iid.to_s, subscribed_state: subscribed_state }
+
+    graphql_mutation(:epic_set_subscription, params)
+  end
+
+  def mutation_response
+    graphql_mutation_response(:epic_set_subscription)
+  end
+
+  context 'when epics feature is disabled' do
+    it_behaves_like 'a mutation that returns top-level errors',
+                    errors: ['The resource that you are attempting to access does not exist '\
+               'or you don\'t have permission to perform this action']
+
+    it 'does not subscribe user to the epic' do
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      expect(epic.subscribed?(current_user)).to be_falsey
+    end
+  end
+
+  context 'when epics feature is enabled' do
+    before do
+      stub_licensed_features(epics: true)
+    end
+
+    context 'when a user wants to subscribe to an epic' do
+      it 'subscribes the user to the epic' do
+        post_graphql_mutation(mutation, current_user: current_user)
+
+        expect(epic.subscribed?(current_user)).to be_truthy
+      end
+    end
+
+    context 'when a user wants to unsubscribe from an epic' do
+      let(:subscribed_state) { false }
+
+      it 'unsubscribes the user from the epic' do
+        epic.subscribe(current_user)
+
+        post_graphql_mutation(mutation, current_user: current_user)
+
+        expect(epic.subscribed?(current_user)).to be_falsey
+      end
+    end
+  end
+end

spec/models/concerns/subscribable_spec.rb

@@ -133,4 +133,60 @@ describe Subscribable, 'Subscribable' do
       end
     end
   end
+
+  describe '#set_subscription' do
+    shared_examples 'setting subscriptions' do
+      context 'when desired_state is set to true' do
+        context 'when a user is subscribed to the resource' do
+          it 'keeps the user subscribed' do
+            resource.subscriptions.create(user: user_1, subscribed: true, project: resource_project)
+
+            resource.set_subscription(user_1, true, resource_project)
+
+            expect(resource.subscribed?(user_1, resource_project)).to be_truthy
+          end
+        end
+
+        context 'when a user is not subscribed to the resource' do
+          it 'subscribes the user to the resource' do
+            expect { resource.set_subscription(user_1, true, resource_project) }
+              .to change { resource.subscribed?(user_1, resource_project) }
+              .from(false).to(true)
+          end
+        end
+      end
+
+      context 'when desired_state is set to false' do
+        context 'when a user is subscribed to the resource' do
+          it 'unsubscribes the user from the resource' do
+            resource.subscriptions.create(user: user_1, subscribed: true, project: resource_project)
+
+            expect { resource.set_subscription(user_1, false, resource_project) }
+              .to change { resource.subscribed?(user_1, resource_project) }
+              .from(true).to(false)
+          end
+        end
+
+        context 'when a user is not subscribed to the resource' do
+          it 'keeps the user unsubscribed' do
+            resource.set_subscription(user_1, false, resource_project)
+
+            expect(resource.subscribed?(user_1, resource_project)).to be_falsey
+          end
+        end
+      end
+    end
+
+    context 'without project' do
+      let(:resource_project) { nil }
+
+      it_behaves_like 'setting subscriptions'
+    end
+
+    context 'with project' do
+      let(:resource_project) { project }
+
+      it_behaves_like 'setting subscriptions'
+    end
+  end
 end