Reply to Blog post activerecord-black-magic

code.rb

# Ref http://spin.atomicobject.com/2012/10/30/activerecord-black-magic/

class EmployeeFinder
  def self.accessible_by(user)
    Employee.joins(:companies) \
      .merge(Company.where(:id => user.company_ids)) \
      .uniq
  end
end

UploadedFile.where(uploaded_by: EmployeeFinder.accessible_by(user))

# Alternately, you could also do this and I think most explain plans will find it more effective

class EmployeeFinder
  def self.accessible_by(user)
    filter = Employee \
      .joins(:companies) \
      .merge(Company.where_company([1,4])) \
      .select(Employee.arel_table[:id])
    Employee.where(:id => filter)
  end
end

# SQL output
>> UploadedFile.by_employees(EmployeeFinder.accessible_by(user))
  UploadedFile Load (0.3ms)  SELECT "uploaded_files".* FROM "uploaded_files" WHERE "uploaded_files"."employee_id" IN (SELECT "employees"."id" FROM "employees" WHERE "employees"."id" IN (SELECT "employees"."id" FROM "employees" INNER JOIN "companies_employees" ON "companies_employees"."employee_id" = "employees"."id" INNER JOIN "companies" ON "companies"."id" = "companies_employees"."company_id" WHERE "companies"."id" IN (1, 4)))