Allow defining :default as lambda

[yogeshjain999]

Allow :default option on property to be accepted as static value or any callable.

property :album, default: "Spring"
property :album, default: ->(represented:, **) { represented.default_album }

[esambo]

I just tried using this in a rails app and updated the Gemfile:

gem "representable", github: "yogeshjain999/representable", branch: "default-lambda"

But I struggle to use it: bin/rails console

Message = Struct.new(:timestamp, :items, keyword_init: true)
Item = Struct.new(:id, :name, keyword_init: true)
class MessageRepresenter < Representable::Decorator
  include Representable::JSON
  nested :metadata, default: {} do
    property :timestamp,
      # default: ->(represented:, **) { represented.timestamp || Time.now.iso8601(3) } # msg.timestamp.call # => ArgumentError (missing keyword: represented)
      # default: ->(options) { Time.now.iso8601(3) } # msg.timestamp.call # => ArgumentError (wrong number of arguments (given 0, expected 1))
      default: -> { Time.now.iso8601(3) } # msg.timestamp.call # works
  end
end

msg = Message.new
MessageRepresenter.new(msg).from_json('{}')
msg.timestamp # => #<Proc:0x00007fd748559818@(irb):9 (lambda)>

It returns a lambda, which is the old behavior on master.
I even tried cloning your repo locally, incremented the version and build a new local gem and then pointed my Gemfile path to it, but that didn't help either.

BTW, it would be really nice to also add some tests for collection and not just property. Collection seem to behave differently if the collection key is present in the input (with an empty array value), or if the key is not present at all.

[yogeshjain999]

Good point @esambo , added tests for collections and nested attributes!

Although, I don't have to explicitly call the procs as per your example 🤔

Can you confirm from your Gemfile.lock that it's using my branch ?

[esambo]

I was able to use this new lambda default in the cloned repo once I switched to the branch and executed:
irb -I lib -r representable:

Message = Struct.new(:timestamp, :items, keyword_init: true)
Item = Struct.new(:id, :name, keyword_init: true)
class MessageRepresenter < Representable::Decorator
  include Representable::JSON
  nested :metadata, default: {} do
    property :timestamp,
      default: ->(represented:, **_options) { Time.now.iso8601(3) }
  end
end

def parsed_timestamp(json)
  msg = Message.new
  MessageRepresenter.new(msg).from_json(json)
  msg.timestamp
end

# works as expected, and returns the current timestamp
parsed_timestamp '{}'
parsed_timestamp '{"metadata":{}}'

# does not work, and will not return the current timestamp
parsed_timestamp '{"metadata":{"timestamp":null}}'
parsed_timestamp '{"metadata":{"timestamp":""}}'
parsed_timestamp '{"metadata":{"timestamp":"NOW"}}'

But my example using a static default in a collection (that works on master) is broken and raises exceptions (in this feature branch):

Message = Struct.new(:timestamp, :items, keyword_init: true)
Item = Struct.new(:id, :name, keyword_init: true)
class MessageRepresenter < Representable::Decorator
  include Representable::JSON
  nested :metadata, default: {} do
    property :timestamp,
      default: :static_default,
      parse_filter: ->(fragment, _options) {
        runtime_default = Time.now.iso8601(3)
        fragment == :static_default ? runtime_default : fragment
      }
  end
  collection :items,
    class: Item,
    default: [{}], # ONLY works if collection key is missing from input. # this default value causes a warning!
    # [Declarative] Defaults#merge! and #call still accept arrays and automatically prepend those. This is now deprecated, you should replace `ary` with `Declarative::Variables::Append(ary)`.
    parse_filter: ->(fragment, options) {
      fragment.empty? ? [Item.new(name: "collection default")] : fragment
  } do
    property :id, default: 0
    nested :details, default: {} do
      property :name, default: "missing"
    end
  end
end

# Empty payload. Defaults work
json = '{}'
msg = Message.new
MessageRepresenter.new(msg).from_json(json)
Time.parse(msg.timestamp).year # => 2022 # GOOD, a dynamic runtime default value in a nested property
msg.items # => [#<struct Item id=0, name="missing">] # GOOD, both have defaults

# Payload with empty collection. Defaults are broken
json = '{"items":[]}'
msg = Message.new
MessageRepresenter.new(msg).from_json(json)
msg.items # => [#<struct Item id=nil, name="collection default">]
msg.items.first.id # => nil # BAD, no default

[esambo]

It would be very helpful if this collection also showed how to use a default:.
BTW, is there a benefit of using instance: ->(*) { Composer.new } instead of class: Composer? The tests seem to pass either way.

[esambo]

It would be helpful if this nested also showed how to use a default:.

[yogeshjain999]

Sorry for the silence in between @esambo, I had some other stuff to look into. I'll reply to above comment soon.

[esambo]

No problem, life can sometimes be very busy.
It seems that these tests are quite complicated and not very descriptive. My impression is that it would be helpful to split up the test setup for different scenarios, and to then explicitly name the tests setup and expected behavior being tested. I would find it very helpful to see all the supported behaviors and their edge cases being tested, which would also help to prevent accidental regression.