Resolve "Migrate issue labels and milestone to related merge request"

app/models/concerns/issuable.rb

@@ -49,7 +49,7 @@ module Issuable
       end
     end
-    has_many :label_links, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+    has_many :label_links, as: :target, dependent: :destroy, inverse_of: :target # rubocop:disable Cop/ActiveRecordDependent
     has_many :labels, through: :label_links
     has_many :todos, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent

app/models/label_link.rb

@@ -3,7 +3,7 @@
 class LabelLink < ActiveRecord::Base
   include Importable
-  belongs_to :target, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
+  belongs_to :target, polymorphic: true, inverse_of: :label_links # rubocop:disable Cop/PolymorphicAssociations
   belongs_to :label
   validates :target, presence: true, unless: :importing?

app/services/issuable_base_service.rb

@@ -129,28 +129,19 @@ class IssuableBaseService < BaseService
     params.merge!(command_params)
   end
-  def create_issuable(issuable, attributes, label_ids:)
-    issuable.with_transaction_returning_status do
-      if issuable.save
-        issuable.update(label_ids: label_ids)
-      end
-    end
-  end
-
   def create(issuable)
     handle_quick_actions_on_create(issuable)
     filter_params(issuable)
     params.delete(:state_event)
     params[:author] ||= current_user
-
-    label_ids = process_label_ids(params)
+    params[:label_ids] = issuable.label_ids.to_a + process_label_ids(params)
     issuable.assign_attributes(params)
     before_create(issuable)
-    if params.present? && create_issuable(issuable, params, label_ids: label_ids)
+    if issuable.save
       after_create(issuable)
       execute_hooks(issuable)
       invalidate_cache_counts(issuable, users: issuable.assignees)

app/services/merge_requests/build_service.rb

@@ -20,6 +20,8 @@ module MergeRequests
       if merge_request.can_be_created
         compare_branches
         assign_title_and_description
+        assign_labels
+        assign_milestone
       end
       merge_request
@@ -135,6 +137,20 @@ module MergeRequests
       append_closes_description
     end
+    def assign_labels
+      return unless target_project.issues_enabled? && issue
+      return if merge_request.label_ids&.any?
+
+      merge_request.label_ids = issue.try(:label_ids)
+    end
+
+    def assign_milestone
+      return unless target_project.issues_enabled? && issue
+      return if merge_request.milestone_id.present?
+
+      merge_request.milestone_id = issue.try(:milestone_id)
+    end
+
     def append_closes_description
       return unless issue&.to_reference.present?
@@ -185,7 +201,9 @@ module MergeRequests
     end
     def issue
-      @issue ||= target_project.get_issue(issue_iid, current_user)
+      strong_memoize(:issue) do
+        target_project.get_issue(issue_iid, current_user)
+      end
     end
   end
 end

app/services/merge_requests/create_from_issue_service.rb

@@ -16,8 +16,6 @@ module MergeRequests
     def execute
       return error('Invalid issue iid') unless @issue_iid.present? && issue.present?
-      params[:label_ids] = issue.label_ids if issue.label_ids.any?
-
       result = CreateBranchService.new(project, current_user).execute(branch_name, ref)
       return result if result[:status] == :error
@@ -58,8 +56,7 @@ module MergeRequests
         source_project_id: project.id,
         source_branch: branch_name,
         target_project_id: project.id,
-        target_branch: ref,
-        milestone_id: issue.milestone_id
+        target_branch: ref
       }
     end

changelogs/unreleased/50657-migrate-issue-labels-and-milestone-to-related-mr-on-creation.yml

+---
+title: Merge request copies all associated issue labels and milestone on creation
+merge_request: 21383
+author:
+type: added

spec/services/merge_requests/build_service_spec.rb

@@ -13,6 +13,8 @@ describe MergeRequests::BuildService do
   let(:description) { nil }
   let(:source_branch) { 'feature-branch' }
   let(:target_branch) { 'master' }
+  let(:milestone_id) { nil }
+  let(:label_ids) { [] }
   let(:merge_request) { service.execute }
   let(:compare) { double(:compare, commits: commits) }
   let(:commit_1) { double(:commit_1, sha: 'f00ba7', safe_message: "Initial commit\n\nCreate the app") }
@@ -25,7 +27,9 @@ describe MergeRequests::BuildService do
                                     source_branch: source_branch,
                                     target_branch: target_branch,
                                     source_project: source_project,
-                                    target_project: target_project)
+                                    target_project: target_project,
+                                    milestone_id: milestone_id,
+                                    label_ids: label_ids)
   end
   before do
@@ -179,6 +183,33 @@ describe MergeRequests::BuildService do
             expect(merge_request.description).to eq(expected_description)
           end
         end
+
+        context 'when the source branch matches an internal issue' do
+          let(:label) { create(:label, project: project) }
+          let(:milestone) { create(:milestone, project: project) }
+          let(:source_branch) { '123-fix-issue' }
+
+          before do
+            issue.update!(iid: 123, labels: [label], milestone: milestone)
+          end
+
+          it 'assigns the issue label and milestone' do
+            expect(merge_request.milestone).to eq(milestone)
+            expect(merge_request.labels).to match_array([label])
+          end
+
+          context 'when milestone_id and label_ids are shared in the params' do
+            let(:label2) { create(:label, project: project) }
+            let(:milestone2) { create(:milestone, project: project) }
+            let(:label_ids) { [label2.id] }
+            let(:milestone_id) { milestone2.id }
+
+            it 'assigns milestone_id and label_ids instead of issue labels and milestone' do
+              expect(merge_request.milestone).to eq(milestone2)
+              expect(merge_request.labels).to match_array([label2])
+            end
+          end
+        end
       end
     end