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Validations with Scrivener (2)

In a previous post I looked at how you might use Scrivener, a minimal validation library for Ruby by Michel Martens, to filter and validate a set of data.

We came across two validation methods or assertions in assert_present and assert_numeric. There are a number of other built-in methods that are all relatively self-explanatory. Here is the full list:

assert_format   # against a Regexp
assert_member   # of an Enum

Each of the methods takes as its first argument a symbol representing the field on which the validation should be applied and, if necessary, a second argument with details of what data should be used as a basis for the validation (e.g. a regular expression or a given length).

All of the above validations are based on a primitive assert method which then also forms the basis for any custom validations that you might wish to write.

To fully understand the assert method we can take a detour into the format of the errors hash. This is a plain Ruby Hash with model attributes as keys mapping to an array of error codes, allowing multiple errors to be logged for any given field:

{ :age =>[:not_numeric] }

Each Scrivener sub-class instantiates this hash in the @errors instance variable and makes it accessible via a method of the same name. Validations are able to modify this hash as necessary.

This is the definition of assert:

def assert(value, error)
  value or errors[error.first].push(error.last) && false

value is an expression that evaluates to a boolean; error is a tuple, masquerading as an two-element array, with the attribute key and the error code, e.g. [:name, :incorrect].

If you look at the implementation of assert_present then you can see how it uses assert under the hood:

def assert_present(att, error = [att, :not_present])
  assert(!send(att).to_s.empty?, error)

It defines a default error of :not_present on the attribute that is passed in, and it asserts that when sending the attribute to self that it isn’t empty.

And these can build on top of each other, for example in assert_format it builds on assert_present:

def assert_format(att, format, error = [att, :format])
  if assert_present(att, error)
    assert(send(att).to_s.match(format), error)

If the value isn’t present then it won’t bother matching the regular expression against the given attributes value.

And assert_decimal builds on assert_format:

DECIMAL = /\A\-?(\d+)?(\.\d+)?\z/

def assert_decimal(att, error = [att, :not_decimal])
  assert_format att, DECIMAL, error

And we therefore have this wonderful Russian Doll style composition of validations built around one primitive method, assert.

It’s easy to see how you might begin to put together your own validations based on these primitives, and that’s what I’ll do in a future post.

The design is incredibly elegant. Simple to understand, simple to extend and compose. It doesn’t get in your way but encourages you to build up a set of cohesive validations, either on the fly in a given Scrivener sub-class, or in a separate module to be included.

Friday 26th February 2021.