Don't pluck IDs when getting issues/MRs per group

This replaces plucking of IDs with a sub-query, saving the overhead of
loading the data in Ruby and then mapping the rows to an Array of IDs.
This also scales much better when dealing with a large amount of IDs
that would be involved.

app/models/issue.rb

@@ -33,7 +33,7 @@ class Issue < ActiveRecord::Base
   belongs_to :project
   validates :project, presence: true
 
-  scope :of_group, ->(group) { where(project_id: group.project_ids) }
+  scope :of_group, ->(group) { where(project_id: group.projects.select(:id)) }
   scope :cared, ->(user) { where(assignee_id: user) }
   scope :open_for, ->(user) { opened.assigned_to(user) }
 

app/models/merge_request.rb

@@ -131,7 +131,7 @@ class MergeRequest < ActiveRecord::Base
   validate :validate_branches
   validate :validate_fork
 
-  scope :of_group, ->(group) { where("source_project_id in (:group_project_ids) OR target_project_id in (:group_project_ids)", group_project_ids: group.project_ids) }
+  scope :of_group, ->(group) { where("source_project_id in (:group_project_ids) OR target_project_id in (:group_project_ids)", group_project_ids: group.projects.select(:id)) }
   scope :by_branch, ->(branch_name) { where("(source_branch LIKE :branch) OR (target_branch LIKE :branch)", branch: branch_name) }
   scope :cared, ->(user) { where('assignee_id = :user OR author_id = :user', user: user.id) }
   scope :by_milestone, ->(milestone) { where(milestone_id: milestone) }