Proposal for decoupling dm-validations from Resource internals

snippet.rb

# TODO: shouldn't Validations use `before :save` to intercept the save call?
#   (for DataMapper::Resource instances, not validated plain Ruby objects)
# 
# Admittedly, it may not be possible to support as 'clean' of an interface
# from the caller's perspective (ie., being able to specify the validation
# context with an arg to the #save/#update methods vs in some other manner).
# 
# However, changing the method signature of #save and #update is problematic.
# Also, I'm not sure how often an explicit validation context is used,
# and on that basis I question whether the interface should be designed for
# the explicit validation context use-case, as it is currently.
# 
# A new interface on the resource could provide a reasonable compromise:

resource.validation_context(:foo).save

# The implementation could be fairly simple, wherein the #validation_context
# call returns a ContextProxy:

module DataMapper::Validations::Resource
  def validation_context(context = Undefined)
    DataMapper::Validations::ContextProxy.new(self, context)
  end
end

# And that, in turn, proxies calls on it into calls much like the current
# implementation (via explicit methods, or #method_missing):

class DataMapper::Validations::ContextProxy
  def save
    Validations::Context.in_context(@context) { @resource.save }
  end
end

# This would allow dm-validations to use a `before :save, :validate` hook to
# intercept the save event and therefore to provide the same functionality as
# currently, but without coupling so tightly with Resource.

# Also, such an architecture would allow preserving (and deprecating) the current
# interface for a transitional period.