How to display Parent record with total number of its Child record in rails

class Attachment < ActiveRecord::Base
  belongs_to :user, foreign_key: :creator_id
  belongs_to :deal_task, foreign_key: :relation_id
end
class DealTask < ActiveRecord::Base
   has_many :attachments, foreign_key: :relation_id
end

I have parent table called DealTask and child table called Attachment

I want a list of DealTask records with associated total number of attachments

I found the best solution for Parent child relationship count

counter_cache: true

because all above queries take too much time to load from database

so you all must prefer to use this

1-> Add one column in Parent table called DealTask

rails g migration AddAttachmentsCountToDealTask attachments_count:integer

2-> Open Migration add Edit it

class AddAttachmentCountToDealTask < ActiveRecord::Migration[5.0]

 def up
  add_column :deal_tasks, :attachments_count, :integer, default: 0
  DealTask.reset_column_information
  DealTask.find_each do |deal_task|
    DealTask.reset_counters deal_task.id, :attachments
  end
 end
 def down
  remove_column :deal_tasks, attachments_count
 end
end

So when you rollback the migration it will not raise an error or exception

you can also use any loop instead of using

find_each, DealTask.all.each do...end

but yes, While resetting counter Must use class name like

DealTask.reset_counters

3-> Set Counter cache

class Attachment < ActiveRecord::Base
 belongs_to :deal_task, foreign_key: :relation_id, counter_cache: true
end
class DealTask < ActiveRecord::Base
 has_many :attachments, foreign_key: :relation_id
end

suppose name of your model is sub_tasks than your counter_cache column must be

sub_tasks_count

if you want your own column name than you have to specify that column name in counter_cache

suppose column name is total_subtasks than

belongs_to :deal_task, foreign_key: :relation_id, counter_cache: :total_subtasks

and make changes accordingly for updating counter_cache

now when you Add any Attachment, attachments_count column increase by 1 and this is done automatically by **counter_cache

one Problem is there ** when you delete any child counter_cache is unable to decrease **

so for that solution make a callback

class Attachment < ActiveRecord::Base
 belongs_to :deal_task, foreign_key: :relation_id, counter_cache: true
 before_destroy :reset_counter
 private
 def reset_counter
  DealTask.reset_counters(self.relation.id, :attachments)
 end
end

so when you delete any attachments it will reset countet_cache for its Parent by relation_id which is parent_id or Foreign_key for attachments

for more info see video on Railscast counter cache 23