validation_test.rb

# Let's say user wants to charge a scooter.
class Charge
  belongs_to :user
  belongs_to :scooter
end

# we want to ensure that scooter is discharged
class Charge
  belongs_to :user
  belongs_to :scooter
  validate do |charge|
    charge.scooter.battery < 0.3 #or whatever magic number
  end
end

Charge.new(user: user, scooter: scooter)

# that makes sense but when scooter changes this object is no longer valid.
# when objects become invalid overtime, things got messed up.

# Point:
# Validation could not depend on external objects.
# When there is more than 1 object in interaction, you need ensure 
# that they could work together as a whole and it should be validated inside
# the application business logic/use case class but not in the model itself.

# Why does this point matter?
# Rails validation is another thing that have a tendency of being overused.