permitted_keys_validator.rb

require 'active_model'

#
# The reusable validator
#
class PermittedKeysValidator < ActiveModel::EachValidator

  def check_validity!
    raise ArgumentError, "you must supply permitted keys" unless permitted_keys.present?
  end

  def validate_each(record, attribute, value)
    value.assert_valid_keys *permitted_keys
  rescue ArgumentError => e
    record.errors.add attribute, e.message
  end

private

  def permitted_keys
    Array(options[:in])
  end

end

#
# Your actual model
#
class Foo
  include ActiveModel::Model

  attr_accessor :my_hash

  validates :my_hash, permitted_keys: [:foo, :bar]

end

#
# Example
#
f = Foo.new(my_hash: {baz: 1}) # => #<Foo @my_hash={:baz=>1}>
f.valid?                       # => false
f.errors.full_messages         # => ["My hash Unknown key: :baz. Valid keys are: :foo, :bar"]

f = Foo.new(my_hash: {bar: 1}) # => #<Foo @my_hash={:bar=>1}>
f.valid?                       # => true

validate_keys.rb

require 'active_model'

class Foo
  include ActiveModel::Model

  attr_accessor :my_hash

  validate :valid_keys

private

  def valid_keys
    @my_hash.assert_valid_keys :foo, :bar
  rescue ArgumentError => e
    errors.add :my_hash, e.message
  end

end

f = Foo.new(my_hash: {baz: 1}) # => #<Foo @my_hash={:baz=>1}>
f.valid?                       # => false
f.errors.full_messages         # => ["My hash Unknown key: :baz. Valid keys are: :foo, :bar"]

f = Foo.new(my_hash: {bar: 1}) # => #<Foo @my_hash={:bar=>1}>
f.valid?                       # => true
#