controller.rb

# Aim: load all activities having the same customer.
#
# PublicActivity::Activity(id: integer, trackable_id: integer, trackable_type: string, ...)
#   => belongs_to :trackable, polymorphic: true
#   => trackable is either Project or Service


PublicActivity::Activity.joins(:trackable)
# ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :trackable


PublicActivity::Activity.includes(:trackable)
#  PublicActivity::Activity Load (0.3ms)  SELECT "activities".* FROM "activities"
#  Project Load (0.2ms)  SELECT "projects".* FROM "projects" WHERE "projects"."id" IN (1)
#  Mandate Load (0.2ms)  SELECT "mandates".* FROM "mandates" WHERE "mandates"."id" IN (1)


PublicActivity::Activity.includes(trackable: :customer)
#  PublicActivity::Activity Load (0.3ms)  SELECT "activities".* FROM "activities"
#  Project Load (0.3ms)  SELECT "projects".* FROM "projects" WHERE "projects"."id" IN (1) 
#  Mandate Load (0.3ms)  SELECT "mandates".* FROM "mandates" WHERE "mandates"."id" IN (1)
#  Customer Load (0.4ms)  SELECT "customers".* FROM "customers" WHERE "customers"."id" IN (1) 
#  Customer Load (0.4ms)  SELECT "customers".* FROM "customers" WHERE "customers"."id" IN (1) 


PublicActivity::Activity.includes(trackable: :customer).where('customers.id = 1').references(:customers)
# ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :trackable

models.rb

class Customer < ActiveRecord::Base
  has_many :projects
  has_many :services
end


class Project < ActiveRecord::Base
  include PublicActivity::Common
  
  belongs_to :customer
end

class Service < ActiveRecord::Base
  include PublicActivity::Common
  
  belongs_to :customer
end