Dynamic relationships

审查之前的讨论和资源关系 API，重点关注提议的类方法或代码块，以及由元数据驱动的图像示例。定义动态公开某个关系的验收标准，使其可以被包含或链接，同时继续支持由应用程序提供的链接；该 issue 没有指定文件或测试，因此请在实现前定位这些入口点。

I am using JR for my project finally. I have a requirement which has been talked about before, which I intend to implement myself, but would like to know if this project is interested in receiving it too. If so, I will develop it on a branch, otherwise I will write a custom extension that will live in my app.

In summary, I require that relationships can also be dynamic rather than hard coded using has_many etc..
The idea being that the client side is helped significantly if we translate something that may end up being either complex to read or maybe would require a number of http requests - into a simple relationship that the client can use easier.

Probably best to use an example

Say we have some metadata in our AR model and that meta data has a definition of "some objects of type 'image' with ids 1,2,3,4,5,6", keyed by the name "my_images". Whilst this could be read by the client, they would then have to make decisions on the type, then go and fetch those images themselves.
We want "my_images" to be a relationship that can be included or linked to as normal (we would have to rely on the application to provide the links though as JR could not possible calculate them itself)

I was thinking of an implementation in the resource as a class method that is passed a block which says "define dynamic relationships" and the block could return maybe an array or relationships to define or something like that - but I haven't thought that much into it yet.