Support creating an MR on a fork from an issue

app/controllers/projects/issues_controller.rb

@@ -172,6 +172,7 @@ class Projects::IssuesController < Projects::ApplicationController
 
   def create_merge_request
     create_params = params.slice(:branch_name, :ref).merge(issue_iid: issue.iid)
+    create_params[:target_project_id] = params[:target_project_id] if Feature.enabled?(:create_confidential_merge_request, @project)
     result = ::MergeRequests::CreateFromIssueService.new(project, current_user, create_params).execute
 
     if result[:status] == :success

app/services/merge_requests/create_from_issue_service.rb

@@ -6,27 +6,30 @@ module MergeRequests
       # branch - the name of new branch
       # ref    - the source of new branch.
 
-      @branch_name = params[:branch_name]
-      @issue_iid   = params[:issue_iid]
-      @ref         = params[:ref]
+      @branch_name       = params[:branch_name]
+      @issue_iid         = params[:issue_iid]
+      @ref               = params[:ref]
+      @target_project_id = params[:target_project_id]
 
       super(project, user)
     end
 
     def execute
+      return error('Project not found') if target_project.blank?
+      return error('Not allowed to create merge request') unless can_create_merge_request?
       return error('Invalid issue iid') unless @issue_iid.present? && issue.present?
 
-      result = CreateBranchService.new(project, current_user).execute(branch_name, ref)
+      result = CreateBranchService.new(target_project, current_user).execute(branch_name, ref)
       return result if result[:status] == :error
 
       new_merge_request = create(merge_request)
 
       if new_merge_request.valid?
-        SystemNoteService.new_merge_request(issue, project, current_user, new_merge_request)
+        SystemNoteService.new_merge_request(issue, target_project, current_user, new_merge_request)
 
         success(new_merge_request)
       else
-        SystemNoteService.new_issue_branch(issue, project, current_user, branch_name)
+        SystemNoteService.new_issue_branch(issue, target_project, current_user, branch_name)
 
         error(new_merge_request.errors)
       end
@@ -34,6 +37,10 @@ module MergeRequests
 
     private
 
+    def can_create_merge_request?
+      can?(current_user, :create_merge_request_from, target_project)
+    end
+
     # rubocop: disable CodeReuse/ActiveRecord
     def issue
       @issue ||= IssuesFinder.new(current_user, project_id: project.id).find_by(iid: @issue_iid)
@@ -45,21 +52,21 @@ module MergeRequests
     end
 
     def ref
-      return @ref if project.repository.branch_exists?(@ref)
+      return @ref if target_project.repository.branch_exists?(@ref)
 
-      project.default_branch || 'master'
+      target_project.default_branch || 'master'
     end
 
     def merge_request
-      MergeRequests::BuildService.new(project, current_user, merge_request_params).execute
+      MergeRequests::BuildService.new(target_project, current_user, merge_request_params).execute
     end
 
     def merge_request_params
       {
         issue_iid: @issue_iid,
-        source_project_id: project.id,
+        source_project_id: target_project.id,
         source_branch: branch_name,
-        target_project_id: project.id,
+        target_project_id: target_project.id,
         target_branch: ref
       }
     end
@@ -67,5 +74,14 @@ module MergeRequests
     def success(merge_request)
       super().merge(merge_request: merge_request)
     end
+
+    def target_project
+      @target_project ||=
+        if @target_project_id.present?
+          project.forks.find_by_id(@target_project_id)
+        else
+          project
+        end
+    end
   end
 end