Validations: Decode response with format

[seanpdoyle]

First, extract the ActiveResource::ErrorsParser class along with the internal ActiveResource::ActiveModelErrorsParser class (that inherits from ActiveResource::ErrorsParser). Configure a errors_parser resource class attribute to control how errors are extracted from decoded payloads.

The errors_parser pattern and the ErrorsParser class are directly inspired by the collection_parser and ActiveResource::Collection class.

ActiveResource::ErrorsParser

ActiveResource::ErrorsParser is a wrapper to handle parsing responses in response to invalid requests that do not directly map to Active Model error conventions.

You can define a custom class that inherits from
ActiveResource::ErrorsParser in order to to set the elements instance.

The initialize method will receive the ActiveResource::Formats parsed result and should set @messages.

Consider a POST /posts.json request that results in a 422 Unprocessable Content response with the following application/json body:

{
  "error": true,
  "messages": ["Something went wrong", "Title can't be blank"]
}

A Post class can be setup to handle it with:

class Post < ActiveResource::Base
  self.errors_parser = PostErrorsParser
end

A custom ActiveResource::ErrorsParser instance's messages method should return a mapping of attribute names (or "base") to arrays of error message strings:

class PostErrorsParser < ActiveResource::ErrorsParser
  def initialize(parsed)
    @messages = Hash.new { |hash, attr_name| hash[attr_name] = [] }

    parsed["messages"].each do |message|
      if message.starts_with?("Title")
        @messages["title"] << message
      else
        @messages["base"] << message
      end
    end
  end
end

When the POST /posts.json request is submitted by calling save, the errors are parsed from the body and assigned to the Post instance's errors object:

post = Post.new(title: "")
post.save                         # => false
post.valid?                       # => false
post.errors.messages_for(:base)   # => ["Something went wrong"]
post.errors.messages_for(:title)  # => ["Title can't be blank"]

If the custom ActiveResource::ErrorsParser instance's messages method returns an array of error message strings, Active Resource will try to infer the attribute name based on the contents of the error message string. If an error starts with a known attribute name, Active Resource will add the message to that attribute's error messages. If a known attribute name cannot be inferred, the error messages will be added to the :base errors:

class PostErrorsParser < ActiveResource::ErrorsParser
  def initialize(parsed)
    @messages = parsed["messages"]
  end
end

Changes to ActiveResource::Formats::JsonFormat and ActiveResource::Formats::XmlFormat

This commit changes the ActiveResource::Errors#from_xml and ActiveResource::Errors#from_json methods to be implemented in terms of a new #from_body method. The #from_body method is flexible enough to support any application-side custom formats, while internally flexible enough to rely on the built-in JSON and XML formats.

[rafaelfranca]

Can you break the changes to ActiveResource::Formats::JsonFormat and ActiveResource::Formats::XmlFormat in another PR? They don't look related with the error parser

[seanpdoyle]

Can you break the changes to ActiveResource::Formats::JsonFormat and ActiveResource::Formats::XmlFormat in another PR? They don't look related with the error parser

Sure! That change is required for this behavior to work, but I'm happy to open a smaller PR to merge it ahead of reviewing this one.

[rafaelfranca]

Yeah, let's break in two. I intended to merge both, but I don't think that change on its own makes sense.

[seanpdoyle]

I've opened #446.