XmlMapper
===========

XmlMapper allows you to parse XML data and convert it quickly and easily into ruby data structures.

This project is a fork of the great work done first by
[jnunemaker](https://github.com/jnunemaker/happymapper).

## Major Differences

  * [Nokogiri](http://nokogiri.org/) support
  * Text nodes parsing
  * Raw XML content parsing
  * `#to_xml` support utilizing the same XmlMapper tags
  * Numerous fixes for namespaces when using composition of classes
  * Fixes for instances of XML where a namespace is defined but no elements with that namespace are found

## Installation

### [Rubygems](https://rubyygems.org/gems/xmlmapper)

    $ gem install xmlmapper

### [Bundler](http://gembundler.com/)
Add the `xmlmapper` gem to your project's `Gemfile`.

    gem 'xmlmapper'

Run the bundler command to install the gem:

    $ bundle install

# Examples

Let's start with a simple example to get our feet wet. Here we have a simple example of XML that defines some address information:

    <address>
      <street>Milchstrasse</street>
      <housenumber>23</housenumber>
      <postcode>26131</postcode>
      <city>Oldenburg</city>
      <country code="de">Germany</country>
    </address>

XmlMapper provides support for simple, zero configuration parsing as well as the ability to model the XML content in classes.

## XmlMapper.parse(XML)

With no classes or configuration you can parse the example XML with little effort:

```ruby
address = XmlMapper.parse(ADDRESS_XML_DATA)
address.street # => Milchstrasse
address.housenumber # => 23
address.postcode # => 26131
address.city # => Oldenburg
address.country.code # => de
address.country.content # => Germany
```

It is important to be aware that this no configuration parsing is limited in capacity:

* All element names are converted to accessor methods with [underscorized](http://rubydoc.info/gems/activesupport/ActiveSupport/Inflector:underscore) names
* All value fields are left as String types
* Determining if there is just one or multiple child elements is hard, so it assumes it is one until it finds another with the same name.

## Address.parse(XML)

XmlMapper will let you easily model this information as a class:

```ruby
require 'xmlmapper'

class Address
  include XmlMapper

  tag 'address'
  element :street, String, :tag => 'street'
  element :postcode, String, :tag => 'postcode'
  element :housenumber, Integer, :tag => 'housenumber'
  element :city, String, :tag => 'city'
  element :country, String, :tag => 'country'
end
```

To make a class XmlMapper compatible you simply `include XmlMapper` within the class definition. This takes care of all the work of defining all the speciality methods and magic you need to get running. As you can see we immediately start using these methods.

* `tag` matches the name of the XML tag name 'address'.

* `element` defines accessor methods for the specified symbol (e.g. `:street`,`:housenumber`) that will return the class type (e.g. `String`,`Integer`) of the XML tag specified (e.g. `:tag => 'street'`, `:tag => 'housenumber'`).

When you define an element with an accessor with the same name as the tag, this is the case for all the examples above, you can omit the `:tag`. These two element declaration are equivalent to each other:

```ruby
element :street, String, :tag => 'street'
element :street, String
```

Including the additional tag element is not going to hurt anything and in some cases will make it absolutely clear how these elements map to the XML. However, once you know this rule, it is hard not to want to save yourself the keystrokes.

Instead of `element` you may also use `has_one`:

```ruby
element :street, String, :tag => 'street'
element :street, String
has_one :street, String
```

These three statements are equivalent to each other.

## Parsing

With the mapping of the address XML articulated in our Address class it is time to parse the data:

```ruby
address = Address.parse(ADDRESS_XML_DATA, :single => true)
puts address.street
```

Assuming that the constant `ADDRESS_XML_DATA` contains a string representation of the address XML data this is fairly straight-forward save for the `parse` method.

The `parse` method, like `tag` and `element` are all added when you included XmlMapper in the class. Parse is a wonderful, magical place that converts all these declarations that you have made into the data structure you are about to know and love.

But what about the `:single => true`? Right, that is because by default when your object is all done parsing it will be an array. In this case an array with one element, but an array none the less. So the following are equivalent to each other:

```ruby
address = Address.parse(ADDRESS_XML_DATA).first
address = Address.parse(ADDRESS_XML_DATA, :single => true)
```

The first one returns an array and we return the first instance, the second will do that work for us inside of parse.

## Multiple Elements Mapping

What if our address XML was a little different, perhaps we allowed multiple streets:

    <address>
      <street>Milchstrasse</street>
      <street>Another Street</street>
      <housenumber>23</housenumber>
      <postcode>26131</postcode>
      <city>Oldenburg</city>
      <country code="de">Germany</country>
    </address>

Similar to `element` or `has_one`, the declaration for when you have multiple elements you simply use:

```ruby
has_many :streets, String, :tag => 'street'
```

Your resulting `streets` method will now return an array.

```ruby
address = Address.parse(ADDRESS_XML_DATA, :single => true)
puts address.streets.join('\n')
```

Imagine that you have to write `streets.join('\n')` for the rest of eternity throughout your code. It would be a nightmare and one that you could avoid by creating your own convenience method.

```ruby
require 'xmlmapper'

class Address
  include XmlMapper

  tag 'address'

  has_many :streets, String

  def streets
    @streets.join('\n')
  end

  element :postcode, String, :tag => 'postcode'
  element :housenumber, String, :tag => 'housenumber'
  element :city, String, :tag => 'city'
  element :country, String, :tag => 'country'
end
```

Now when we call the method `streets` we get a single value, but we still have the instance variable `@streets` if we ever need to the values as an array.


## Attribute Mapping

    <address location='home'>
      <street>Milchstrasse</street>
      <street>Another Street</street>
      <housenumber>23</housenumber>
      <postcode>26131</postcode>
      <city>Oldenburg</city>
      <country code="de">Germany</country>
    </address>

Attributes are absolutely the same as `element` or `has_many`

```ruby
attribute :location, String, :tag => 'location
```

Again, you can omit the tag if the attribute accessor symbol matches the name of the attribute.


### Attributes On Empty Child Elements

    <feed xml:lang="en-US" xmlns="http://www.w3.org/2005/Atom">
      <id>tag:all-the-episodes.heroku.com,2005:/tv_shows</id>
      <link rel="alternate" type="text/html" href="http://all-the-episodes.heroku.com"/>
      <link rel="self" type="application/atom+xml" href="http://all-the-episodes.heroku.com/tv_shows.atom"/>
      <title>TV Shows</title>
      <updated>2011-07-10T06:52:27Z</updated>
    </feed>

In this case you would need to map an element to a new `Link` class just to access `<link>`s attributes, except that there is an alternate syntax. Instead of

```ruby
class Feed
  # ....
  has_many :links, Link, :tag => 'link', :xpath => '.'
end

class Link
  include XmlMapper

  attribute :rel, String
  attribute :type, String
  attribute :href, String
end
```

You can drop the `Link` class and simply replace the `has_many` on `Feed` with

```ruby
element :link, String, :single => false, :attributes => { :rel => String, :type => String, :href => String }
```

As there is no content, the type given for `:link` (`String` above) is irrelevant, but `nil` won't work and other types may try to perform typecasting and fail. You can omit the :single => false for elements that only occur once within their parent.

This syntax is most appropriate for elements that (a) have attributes but no content and (b) only occur at only one level of the heirarchy. If `<feed>` contained another element that also contained a `<link>` (as atom feeds generally do) it would be DRY-er to use the first syntax, i.e. with a separate `Link` class.


## Class composition (and Text Node)

Our address has a country and that country element has a code. Up until this point we neglected it as we declared a `country` as being a `String`.

    <address location='home'>
      <street>Milchstrasse</street>
      <street>Another Street</street>
      <housenumber>23</housenumber>
      <postcode>26131</postcode>
      <city>Oldenburg</city>
      <country code="de">Germany</country>
    </address>

Well if we only going to parse country, on it's own, we would likely create a class mapping for it.

```ruby
class Country
  include XmlMapper

  tag 'country'

  attribute :code, String
  content :name, String
end
```

We are utilizing an `attribute` declaration and a new declaration called `content`.

* `content` is used when you want the text contained within the element

Awesome, now if we were to redeclare our `Address` class we would use our new `Country` class.

```ruby
class Address
  include XmlMapper

  tag 'address'

  has_many :streets, String, :tag => 'street'

  def streets
    @streets.join('\n')
  end

  element :postcode, String, :tag => 'postcode'
  element :housenumber, String, :tag => 'housenumber'
  element :city, String, :tag => 'city'
  element :country, Country, :tag => 'country'
end
```

Instead of `String`, `Boolean`, or `Integer` we say that it is a `Country` and XmlMapper takes care of the details of continuing the XML mapping through the country element.

```ruby
address = Address.parse(ADDRESS_XML_DATA, :single => true)
puts address.country.code
```

A quick note, in the above example we used the constant `Country`. We could have used `'Country'`. The nice part of using the latter declaration, enclosed in quotes, is that you do not have to define your class before this class. So Country and Address can live in separate files and as long as both constants are available when it comes time to parse you are golden.

## Custom XPATH

### Has One, Has Many

Getting to elements deep down within your XML can be a little more work if you did not have xpath support. Consider the following example:

    <media>
      <gallery>
        <title href="htttp://fishlovers.org/friends">Friends Who Like Fish</title>
        <picture>
          <name>Burtie Sanchez</name>
          <img>burtie01.png</img>
        </picture>
      </gallery>
      <picture>
        <name>Unsorted Photo</name>
        <img>bestfriends.png</img>
      </picture>
    </media>

You may want to map the sub-elements contained buried in the 'gallery' as top level items in the media. Traditionally you could use class composition to accomplish this task, however, using the xpath attribute you have the ability to shortcut some of that work.

```ruby
class Media
  include XmlMapper

  has_one :title, String, :xpath => 'gallery/title'
  has_one :link, String, :xpath => 'gallery/title/@href'
end
```

## Shared Functionality

### Inheritance Approach

While mapping XML to objects you may arrive at a point where you have two or more very similar structures.

```ruby
class Article
  include XmlMapper

  has_one :title, String
  has_one :author, String
  has_one :published, Time

  has_one :entry, String

end

class Gallery
  include XmlMapper

  has_one :title, String
  has_one :author, String
  has_one :published, Time

  has_many :photos, String

end
```

In this example there are definitely two similarities between our two pieces of content. So much so that you might be included to create an inheritance structure to save yourself some keystrokes.

```ruby
class Content
  include XmlMapper

  has_one :title, String
  has_one :author, String
  has_one :published, Time
end

class Article < Content
  include XmlMapper

  has_one :entry, String
end

class Gallery < Content
  include XmlMapper

  has_many :photos, String
end
```

### Module Mixins Approach

You can also solve the above problem through mixins.

```ruby
module Content
  def self.included(content)
    content.has_one :title, String
    content.has_one :author, String
    content.has_one :published, Time
  end

  def published_time
    @published.strftime("%H:%M:%S")
  end
end

class Article
  include XmlMapper

  include Content
  has_one :entry, String
end

class Gallery
  include XmlMapper

  include Content
  has_many :photos, String
end
```

Here, when we include `Content` in both of these classes the module method `#included` is called and our class is given as a parameter. So we take that opportunity to do some surgery and define our xmlmapper elements as well as any other methods that may rely on those instance variables that come along in the package.


## Filtering with XPATH (non-greedy)

I ran into a case where I wanted to capture all the pictures that were directly under media, but not the ones contained within a gallery.

    <media>
      <gallery>
        <picture>
          <name>Burtie Sanchez</name>
          <img>burtie01.png</img>
        </picture>
      </gallery>
      <picture>
        <name>Unsorted Photo</name>
        <img>bestfriends.png</img>
      </picture>
    </media>

The following `Media` class is where I started:

```ruby
require 'xmlmapper'

class Media
  include XmlMapper

  has_many :galleries, Gallery, :tag => 'gallery'
  has_many :pictures, Picture, :tag => 'picture'
end
```

However when I parsed the media xml the number of pictures returned to me was 2, not 1.

```ruby
pictures = Media.parse(MEDIA_XML,:single => true).pictures
pictures.length.should == 1   # => Failed Expectation
```

The reason that 2 elements are returned and not 1 is because the default
mappings are assigned XPATH './/' which makes them greedy. Essentially by
default it will find all elements with the tag 'pictures' at the current
level of the document and anywhere else within the document.

To limit an element from being greedy and only finding elements at the
level of the current node you can specify an XPATH.

```ruby
has_many :pictures, Picture, :tag => 'picture', :xpath => '.'
```

`.` states that we are only interested in pictures that can be found directly
under the current node. So when we parse again we will have only our one element.


## Namespaces

Obviously your XML and these trivial examples are easy to map and parse because they lack the treacherous namespaces that befall most XML files.

Perhaps our `address` XML is really swarming with namespaces:

    <prefix:address location='home' xmlns:prefix="http://www.unicornland.com/prefix">
      <prefix:street>Milchstrasse</prefix:street>
      <prefix:street>Another Street</prefix:street>
      <prefix:housenumber>23</prefix:housenumber>
      <prefix:postcode>26131</prefix:postcode>
      <prefix:city>Oldenburg</prefix:city>
      <prefix:country code="de">Germany</prefix:country>
    </prefix:address>

Here again is our address example with a made up namespace called `prefix` that comes direct to use from unicornland, a very magical place indeed. Well we are going to have to do some work on our class definition and that simply adding this one liner to the `Address` class:

```ruby
class Address
  include XmlMapper

  tag 'address'
  namespace 'prefix'
  # ... rest of the code ...
end
```

Of course, if that is too easy for you, you can append a `:namespace => 'prefix` to every one of the elements that you defined.

```ruby
has_many :street, String, :tag => 'street', :namespace => 'prefix'
element :postcode, String, :tag => 'postcode', :namespace => 'prefix'
element :housenumber, String, :tag => 'housenumber', :namespace => 'prefix'
element :city, String, :tag => 'city', :namespace => 'prefix'
element :country, Country, :tag => 'country', :namespace => 'prefix'
```

I definitely recommend the former, as it saves you a whole hell of lot of typing. However, there are times when appending a namespace to an element declaration is important and that is when it has a different namespace then `namespsace 'prefix'`.

Imagine that our `country` actually belonged to a completely different namespace.

    <prefix:address location='home' xmlns:prefix="http://www.unicornland.com/prefix"
    xmlns:different="http://www.trollcountry.com/different">
      <prefix:street>Milchstrasse</prefix:street>
      <prefix:street>Another Street</prefix:street>
      <prefix:housenumber>23</prefix:housenumber>
      <prefix:postcode>26131</prefix:postcode>
      <prefix:city>Oldenburg</prefix:city>
      <different:country code="de">Germany</different:country>
    </prefix:address>

Well we would need to specify that namespace:

```ruby
element :country, Country, :tag => 'country', :namespace => 'different'
```

With that we should be able to parse as we once did.

## Large Datasets (in_groups_of)

When dealing with large sets of XML that simply cannot or should not be placed into memory the objects can be handled in groups through the `:in_groups_of` parameter.

```ruby
Address.parse(LARGE_ADDRESS_XML_DATA,:in_groups_of => 5) do |group|
  puts address.streets
end
```

This trivial block will parse the large set of XML data and in groups of 5 addresses at a time display the streets.

## Saving to XML

Saving a class to XML is as easy as calling `#to_xml`.  The end result will be the current state of your object represented as xml. Let's cover some details that are sometimes necessary and features present to make your life easier.


### :on_save

When you are saving data to xml it is often important to change or manipulate data to a particular format. For example, a time object:

```ruby
has_one :published_time, Time, :on_save => lambda {|time| time.strftime("%H:%M:%S") if time }
```

Here we add the options `:on_save` and specify a lambda which will be executed on the method call to `:published_time`.

### :state_when_nil

When an element contains a nil value, or perhaps the result of the :on_save lambda correctly results in a nil value you will be happy that the element will not appear in the resulting XML. However, there are time when you will want to see that element and that's when `:state_when_nil` is there for you.

```ruby
has_one :favorite_color, String, :state_when_nil => true
```

The resulting XML will include the 'favorite_color' element even if the favorite color has not been specified.

### :read_only

When an element, attribute, or text node is a value that you have no interest in
saving to XML, you can ensure that takes place by stating that it is `read only`.

```ruby
has_one :modified, Boolean, :read_only => true
attribute :temporary, Boolean, :read_only => true
```

This is useful if perhaps the incoming XML is different than the out-going XML.

### namespaces

Parsing the XML to objects only required you to simply specify the prefix of the namespace you wanted to parse, when you persist to xml you will need to define your namespaces so that they are correctly captured.

```ruby
class Address
  include XmlMapper

  register_namespace 'prefix', 'http://www.unicornland.com/prefix'
  register_namespace 'different', 'http://www.trollcountry.com/different'

  tag 'address'
  namespace 'prefix'

  has_many :street, String
  element :postcode, String
  element :housenumber, String
  element :city, String
  element :country, Country, :tag => 'country', :namespace => 'different'

end
```