Allow a default value when the attribute is assigned as nil, with the :allow_nil option

[JanDintel]

This MR adds the option :allow_nil to a Virtus::Attribute. The option allows the user to specify wether to use the default value of an attribute that got assigned as nil. The option will be false by default for backwards compatibility.

See gist for before and after examples

Related issues for this MR are: #291 and #172.

[coveralls]

Coverage increased (+0.01%) when pulling a75ba02 on JanDintel:allow-nil-option into 5093278 on solnic:master.

[JanDintel]

Could you have a look at this MR @elskwid?

[JanDintel]

Maybe you could have a look @solnic?

[solnic]

I will take a look yes! My initial reaction is "OMG NO" but maybe I'm missing something, I need to take a closer look. Will get back to you as soon as possible.

[JanDintel]

@solnic Maybe it's just the name of the option (:allow_nil) that puts you off. But I think this feature/fix is a good addition to Virtus, while staying backwards compatible in the API.

[jpmoral]

👍 This also seems related to #235.

[booch]

I don't think the example should use a default, because it's much less likely that you'd need to worry about nil if you've got a default.

[booch]

Oh, you're suggesting to use the default if nil is passed in. Hmm. That's a harder sell.

[envygeeks]

Why do you need :allow_nil => false when you send a default?

[JanDintel]

@envygeeks Maybe this example explains it the best:

class Page
  include Virtus.model

  attribute :title, String, :default => 'New page', :allow_nil => true
  attribute :author, String, :default => 'Mies', :allow_nil => false
end

page = Page.new(title: nil, author: nil)
page.title     # => nil
page.author    # => 'Mies'

With the :allow_nil => false option, the default is used when the attributes is assigned as nil.

Edit: Just saw that I published a gist that describes the behaviour as well.

[booch]

Can you talk more about the differences between page = Page.new(title: nil, author: nil) and page = Page.new?

[booch]

I'm inclined to accept this, or something similar. Since we don't do validations, we can't just reject the whole object if one attribute contains something that we really don't want in our object. So at this level, I suppose it makes sense to ensure our object is in a useable state, whatever that might mean.

[lime]

I've encountered this need a couple of times; it would be great to be able to specify that an attribute falls back to the default when set to nil.

I wonder if this would be an easier feature to pull through if the naming was more explicit? I could see how :allow_nil could be misinterpreted as to mean something different.

Something like :fallback_to_default_on_nil would be hard to misunderstand, though it's quite a mouthful. Would maybe :default_on_nil be close enough..?

[taryneast]

Bump... I'd really like this functionality