Hello,
ActiveResource currently uses ActiveSupport's Hash extension's definition of from_xml (which in turn just wraps XmlSimple right now) to parse up XML returned from a serving application.
In Rails, the "serving application" (the application exporting a resource) will typically call to_xml on the concerned AR object to generate a chunk of XML which is then picked up by a client ActiveResource-using application.
The issue is that to_xml will do something like this when called on an Array (i.e., when rendering XML for a collection, say):
<entities> <entity> <id>1</id> ... </entity> <entity> <id>2</id> ... </entity> ... </entities>
On the ActiveResource side, Hash's from_xml will right now, due to XmlSimple, present this as follows (sorry for the pseudo-codish syntax here):
{ 'entities' => { 'entity' => [ #Instances_of_Hash ] } }
Wht's wrong with this? According to XmlSimple, nothing. However, given the implementation of to_xml, which I think makes a lot of sense, it would arguably make more sense for from_xml to return this:
{ 'entities' => [ #Instances_of_Hash ] }
Then the collection itself, called 'entities', can easily get logically mapped by ARes to an Array of Entity ARes instances. This makes more sense for ARes, I think.
Similarly, consider a resource with a nested collection, as follows:
<entity> <id>1</id> ...other attribs...
<some-other-entities> <some-other-entity> <id>1</id> ... </some-other-entity> <some-other-entity> <id>2</id> ... </some-other-entity> ... </some-other-entities> </entity>
from_xml will turn this into something like:
{ 'entity' => { 'id' => 1, ... 'some-other-entities' => { 'some-other-entity' => [ #Hash ] } } }
What's undesirable here, in my opinion, is the unnecessary "some-other-entities"->"some-other-entity" nesting. Here's a more preferable representation:
{ 'entity' => { 'id' => 1, ... 'some-other-entities' => [ #Instances_of_Hash ] } }
There are also issues with how XmlSimple in its current use in from_xml deals with single-element arrays.
I corrected the issues with ARes handling of collections as it stands and wrote some tests. The ticket in question is: http://dev.rubyonrails.org/ticket/6291 -- the latest patch in the ticket, activeresource_collections_ALTERNATE_with_tests.diff, is what you should be looking at.
The particularity in my current implementation is that I define a "fixup" method for ARes, in ARes Connection class, to manipulate the Hash returned by Hash.from_xml(), into something which I think is much saner for ARes, given the way to_xml works.
Arguably, and this is where I hope you all chime in, this "fixing up" actually rightfully belongs in ActiveSupport's extension of Hash, that is directly in from_xml. However, there are at least two outstanding issues in my mind with doing this:
1) There are other consumers of Hash.from_xml() -- both in Rails itself, and maybe in outside plugins. I haven't investigated all of them yet, and frankly I'm not sure if they would have to be changed as well;
2) Perhaps more concerning, and partly related to (1), is the fact that there are some edge cases which I'm not sure how to handle in the general Hash.from_xml case, but which make sense in ARes. Namely, how to handle:
<entities> <entity> <id>1</id> ... </entity> </entities>
(i.e., the case where we've got a collection with a single entity). XmlSimple's 'forcearray' is currently set to false in Hash.from_xml, but for ARes it would make more sense for it to be true, and then if "entity".pluralize == parent_node_name (true in the above example), then "pullup" the entity collection. Not sure if my point is getting across here, so please ask me to elaborate if necessary.
Question: commit ticket 6291 as it stands for now since it fixes some collection handling edge case bugs in ARes, refactor 6291, or push 6291's idea in a more general way into ActiveSupport's Hash.from_xml?
Thanks,
