group member count optimization

db/migrate/20190722003541_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class AddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  def up
+    add_column :groups, :member_count, :integer
+    change_column_default :groups, :member_count, 0
+  end
+
+  def down
+    remove_column :groups, :member_count
+  end
+end

db/migrate/20190722003649_backfill_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class BackfillAddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    Group.in_batches do |relation|
+      relation.update_all member_count: 0
+      sleep(0.1)
+    end
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_07_21_234917) do
+ActiveRecord::Schema.define(version: 2019_07_22_003649) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -358,6 +358,7 @@ ActiveRecord::Schema.define(version: 2019_07_21_234917) do
     t.boolean "is_archived", default: false, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "member_count", default: 0
     t.index ["account_id"], name: "index_groups_on_account_id"
   end