validation_context.rb

# default context :create when new, :update when persisted.
# Since rails 4.2 you can define add multipe contexts per validation
# Since rails 5.0 you can specify multiple context when saving/validating.

class Invoice < ApplicationRecord
  validate :check_remote, on: [:create, :remote]
  validate :check_total, on: [:create]
  validate :foo, on: :remote

  def check_remote
     errors.add :base, 'remote not right'
  end

  def check_total
     errors.add :total, 'total not right'
  end

  def foo
    errors.add :base, 'bar not right'
  end
end

invoice = Invoice.new
invoice.save # ["remote not right", "Total total not right"]
# specifying a context overwrites the default context.
invoice.save context: :remote # ["remote not right", "bar not right"]
# so add it in when needed.
invoice.save context: [:remote, :create] # ["remote not right", "bar not right", "Total total not right"]

# valid works the save as save.
invoice.valid? # ["remote not right", "Total total not right"] 
invoice.valid? :remote # ["remote not right", "bar not right"]
invoice.valid? [:remote, :create] # ["remote not right", "bar not right", "Total total not right"] 

invoice = Invoice.last
invoice.save # correct, since we're doing nothing on: :update
invoice.save context: :remote # ["remote not right", "bar not right"]
invoice.valid? [:create, :remote] # ["remote not right", "bar not right", "Total total not right"]