Decontaminate

Decontaminate is a tool for extracting information from large, potentially nested XML documents. It provides a simple Ruby DSL for selecting values from Nokogiri objects and storing the results in JSON-like Ruby hashes and arrays.

Add this line to your application's Gemfile:

gem 'decontaminate'

And then execute:

$ bundle

Or install it yourself as:

$ gem install decontaminate

Decontaminate provides a DSL for creating decontaminators, which, when instantiated, accept XML nodes or documents and produce a hash as a result. To start, create a class that inherits from Decontaminate::Decontaminator:

class MyDecontaminator < Decontaminate::Decontaminator
end

If parsing an entire document, you should specify the name of the root element:

class MyDecontaminator < Decontaminate::Decontaminator
  self.root = 'User'
end

To select values from the XML document, use the scalar class method:

class MyDecontaminator < Decontaminate::Decontaminator
  self.root = 'User'

  scalar 'Name'
  scalar 'Age', type: :integer
  scalar 'DateRegistered', key: 'registered_at'
end

This might produce a result like the following:

=> MyDecontaminator.new(xml_document).as_json
{
  'name' => 'Jane Smith',
  'age' => 28,
  'registered_at' => '2013-08-16T20:51:34.236Z'
}

The first argument to scalar is the name of the node to extract data from. In fact, this can be any XPath string relative to the document root. By default, the resulting JSON key is inferred from the provided path, but it can also be overridden with the key: argument. Additionally, the type of the scalar can be specified with the type: argument, which defaults to :string.

Attributes can be specified with XPath syntax by prepending an @ sign:

scalar '@id', type: :integer

In addition to customization of the parser using the type: keyword argument, scalar can be provided with a block that will allow custom transformation of the value. It will be supplied with the value as parsed according to the provided type, and the return value will be the result stored in the output.

scalar 'RatingPercentage', key: 'rating_ratio', type: :float do |percentage|
  percentage && percentage / 100.0
end

Transformer blocks are evaluated in the context of the decontaminator instance, so instance methods can be called. Additionally, it is possible to call instance methods as transformers directly without needing to pass a block by passing the name of the method as the transformer: keyword argument.

scalar 'RatingPercentage',
       key: 'rating_ratio',
       type: :float,
       transformer: :percentage_to_ratio

def percentage_to_ratio(percentage)
  percentage && percentage / 100.0
end

It's also possible to specify nested or even deeply nested hashes with the hash class method:

hash 'UserProfile', key: 'profile' do
  scalar 'Description'

  hash 'Specialization' do
    scalar 'Area'
    scalar 'Expertise', type: :float
  end
end

The hash method accepts a block, which works just like the class body, but all paths are scoped to the path passed to hash. The key argument is optional, just like with scalar.

Sometimes it may be useful to create an additional hash in the output as an organizational tool, even though there is no equivalent nesting in the input XML. In this case, the XPath argument may be omitted, specifying only key:.

hash key: 'info' do
  scalar 'Email'
end

This will fetch a value from the Email node on the root, but it will be stored in a property within a separate hash, keyed in the result with 'info'.

In addition to the scalar and hash methods, there are plural forms which allow parsing and extracting data that appears many times within a single document. These are named scalars and hashes, respectively. They work much like their singular counterparts, but the provided path should match multiple elements.

For example, given the following decontaminator:

class ArticlesDecontaminator < Decontaminate::Decontaminator
  hashes 'Articles' do
    scalar 'Name'
    scalars 'Tags'
  end
end

And given the following XML document:

<Articles>
  <Article>
    <Name>Article A</Name>
    <Tags>
      <Tag>News</Tag>
      <Tag>Technology</Tag>
    </Tags>
  </Article>
  <Article>
    <Name>Article B</Name>
    <Tags>
      <Tag>Sports</Tag>
      <Tag>Recreation</Tag>
    </Tags>
  </Article>
</Articles>

The resulting object will have the following structure:

{
  'articles' => [
    {
      'name' => 'Article A',
      'tags' => ['News', 'Technology']
    },
    {
      'name' => 'Article B',
      'tags' => ['Sports', 'Recreation']
    }
  ]
}

There are some special things to note in the above example:

• The name of the individual elements is inferred from the parent key.

  In both cases, the parent element was the plural form of its children (Articles/Article and Tags/Tag). Since this is common, the plural forms automatically perform this name inference.

  Since this behavior is sometimes unwanted, it can be disabled by passing the path as an explicit path: keyword argument.

  scalars path: 'Tags/TagName', key: 'tags' # Performs no name inference

• No root element was specified since the root element is a plural.

  When using name inference for a plural element at the root, specifying the root element is an error. By using the explicit path: form mentioned above, root could still be specified.

  self.root = 'Articles'
  hashes path: 'Article', key: 'articles' do; ...; end

Complementing scalar and hash is tuple, which accepts multiple paths and returns a fixed-length array containing an element for each path.

tuple ['Height/text()', 'Height/@units'], key: 'height_with_units'

The tuple method is most useful when supplied with a block, which works like scalar's value transformer, but is supplied with an argument for each path. This allows values to be parsed from multiple values in the source document.

tuple ['Height/text()', 'Height/@units'], key: 'height_cm' do |height, units|
  convert_units height.to_f, from: units, to: 'cm'
end

Tuples also support the shorthand transformer: argument that scalar and scalars support.

Since source data is sometimes more nested than is desired, the with method is a helper for scoping decontamination directives to a given XML element without increasing the nesting depth of the resulting object. Like hash, it accepts an XPath and a block, but the attributes created from within the block will not be wrapped in a hash.

with 'Some/Nested/Data' do
  scalar 'Value'
end

There is no plural form for with since it would, by necessity, create duplicate keys.