# Check if a field value is already used
#
# This class provides a stub validator for fields that should contain a unique value along an
# application (typically logins or ids).
#
# Here an example that uses a `Repository` if an email is unique:
# ~~~nitish
# class UniqueEmailField
#	super UniqueField
#
#	var users: UsersRepository
#
#	redef fun check_unicity(v, field, val) do
#		var user = users.find_by_email(val)
#		if user != null then
#			v.validation.add_error(field, "Email `{val}` already used")
#			return false
#		end
#		return true
#	end
# end
# ~~~
class UniqueField
	super StringField

	# Check if `val` is already used somewhere
	#
	# You must redefine this method to handle your own validation.
	fun check_unicity(v: ObjectValidator, field, val: String): Bool is abstract

	redef fun validate_field(v, obj) do
		if not super then return false
		var val = obj.get_or_null(field)
		if not val isa String then return false
		if not check_unicity(v, field, val) then return false
		return true
	end
end