HappyMapper
Happymapper 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.
Major Differences
- Nokogiri support
- Text nodes parsing
- Raw XML content parsing
#to_xml
support utilizing the same HappyMapper 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
$ gem install nokogiri-happymapper
Bundler
Add the nokogiri-happymapper
gem to your project's Gemfile
.
gem 'nokogiri-happymapper', :require => 'happymapper'
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>
Happymapper provides support for simple, zero configuration parsing as well as the ability to model the XML content in classes.
HappyMapper.parse(XML)
With no classes or configuration you can parse the example XML with little effort:
address = HappyMapper.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 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)
Happymapper will let you easily model this information as a class:
require 'happymapper'
class Address
include HappyMapper
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 HappyMapper compatible you simply include HappyMapper
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:
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
:
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:
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 HappyMapper 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:
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:
has_many :streets, String, :tag => 'street'
Your resulting streets
method will now return an array.
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.
require 'happymapper'
class Address
include HappyMapper
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
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
class Feed
# ....
has_many :links, Link, :tag => 'link', :xpath => '.'
end
class Link
include HappyMapper
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
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.
class Country
include HappyMapper
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.
class Address
include HappyMapper
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 HappyMapper takes care of the details of continuing the XML mapping through the country element.
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.
class Media
include HappyMapper
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.
class Article
include HappyMapper
has_one :title, String
has_one :author, String
has_one :published, Time
has_one :entry, String
end
class Gallery
include HappyMapper
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.
class Content
include HappyMapper
has_one :title, String
has_one :author, String
has_one :published, Time
end
class Article < Content
include HappyMapper
has_one :entry, String
end
class Gallery < Content
include HappyMapper
has_many :photos, String
end
Module Mixins Approach
You can also solve the above problem through mixins.
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 HappyMapper
include Content
has_one :entry, String
end
class Gallery
include HappyMapper
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 happymapper 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:
require 'happymapper'
class Media
include HappyMapper
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.
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.
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:
class Address
include HappyMapper
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.
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:
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.
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:
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.
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
.
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.
class Address
include HappyMapper
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