How should I handle HATEOAS links and references in JSON?

I'm in the process of designing a REST api and to be as RESTful as it gets. I want to incorporate HATEOAS into the json responses.

Adding URLs to related resources is easy enough, but there was some discussion over the structure to use for those links.

A LOT of articles I found use a structure borrowed from ATOM feeds:

"links": [ 
    {"rel": "self", "href":"http://example.org/entity/1"},
    {"rel": "friends", "href":"http://example.org/entity/1/friends"}, ... 
]

This raised some questions:

Why use an array as a container? According to a javascript developer I know, access to the links would be easier with the links as properties of an object. For example:
```
"self":    { "href":"http://example.org/entity/1" }, /* (facebook uses this) */  
"friends": { "href":"http://example.org/entity/1/friends", "type": "..."}
```
Is there a common json structure (beside adapting atom again) to describe references in resource properties? (for example the sender of a message).

The reference should probably be resolved as a url again, but would it be bad to include the simple id as well? kind of like:
```
"sender": { 
    "id": 12345,
    "href": "resource-uri"
}
```

My way of thought is that while HATEOAS makes it so a client doesn't need a lot of knowledge to use an API, I'm kind of reluctant to remove the possibility to USE that knowledge (like accessing the profile picture by building the link client-side without looking up the user first).

Solution

I restarted this topic on the API-Craft google group and got some great responses.

The main advantages of the Array design are:

multiple links for the same relationship
multiple relationships for the same link without writing the link aggain
the ability to order the links

The map of cause has better accessibility.

As far as structure goes there are a lot of possibilities:

JSON-HAL: http://blog.stateless.co/post/13296666138/json-linking-with-hal or also http://stateless.co/hal_specification.html
JSON-LD: http://json-ld.org/ (optionally using Hydra vocabulary)
JSON-Schema: http://json-schema.org/ (theHyper Meta-Schema on the bottom of the page)
Collection+JSON: http://amundsen.com/media-types/collection/

I guess i will go with HAL as it is the cleanest solution, the rest all look kind of... strange for json.