Allows attribute type to be specified as Array[Person]

[krisleech]

Opening this PR to start discussion.

I would like to be able to do this:

class Person
  attr_reader :name

  def initalize(attributes); 
    @name = attributes.fetch(:name)
  end
end

class MyForm
  include Lotus::Validations

  attribute :people, type: Array[Person]
end

form = MyForm.new(people: [{ name: 'Kris', name: 'Lindsey' }]
form.people.first.name # => 'Kris'
form.people.all? { |p| p.is_a?(Person) } # => true

This PR is a quick hack, there are no tests. If you think it is an okay direction - handling a coercer which is a one element Array, as a special case - then I will submit a proper PR with tests.

In theory you could also support something like type: Array[Person, Fruit, Mineral] if the collection was not homogeneous but can't see a usecase for this and it would probably be confusing.

[krisleech]

Discussion here: https://discuss.lotusrb.org/t/allow-lotus-validations-to-coerce-members-of-an-array/120

[duncanita]

Hi, I have problem with validation of params like this

{ users: [ { name: 'bob'}, {name: 'Joe'}] }

[jodosha]

@krisleech @duncanita I've looked closely at this issue and for me is a 👎

What would that syntax mean in terms of code? Would that Array[<Object>] be an object that could be validated (respond to #valid?) or is just a type notation?

If it's the first case, how to handle the error messages with the "dot syntax" for attributes (eg signup.user.address.city)? If you receive { people: [ {name: 'Alice'}, { name: '' } ]}, how to return the error message for the single wrong entry?

If it's just a type notation, should the validation pass if we're able to coerce it? Imagine we have a validation like this: dates: Array[Date], would data like: { dates: ['2020-01-01'] } be valid? Theoretically yes, but it lacks of flexibility because you may also want to add further validation rules.

I think this should be implemented via a custom validation. See #72