apply_filter and apply_sort respects :delegate option of attribute

[senid231]

Allows to sort and filter by delegated attribute without additional patching.
For example request like

get '/posts', params: {
    filter: {name: 'some_title'}, 
    sort: '-name', 
    fields: {posts: 'name'}
}

for such resource

class PostResource < JSONAPI::Resource
  model_name 'Post'

  attribute :name, delegate: :title
  attribute :body

  filter :name
end

will responds successfully with correct data.

[senid231]

maybe we should also change keys in model_error_messages

[senid231]

@lgebhardt, @dgeb, @barelyknown, what do you think about this?

[hidde-jan]

The fact that the two code changes are exactly the same shows that there needs to be a better solution to mapping resource attributes to model attributes and vice versa. Like @senid231 mentions, a proper solution should probably also take care of validation messages (and the corresponding pointers). Something on a class level like

class ProductResource < JSONAPI::Resource
  attribute :name, delegate: :title
  attribute :price

  def self.model_attribute_name(resource_attribute)
    # :name => :title | :price => :price
  end

  def self.resource_attribute_name(model_attribute)
    # :title => :name | :price => :price
  end
end

[senid231]

rebased and dried
@lgebhardt please take a look

[lgebhardt]

@senid231 Thanks!