Allow setting a custom method/property as the primary key (_id)

[rubenrails]

Currently, there are a couple of ways of achieving this, either by overriding the id method entirely, or by using a callback (e.g. before_save).

It'd be better if we could do something like:
Option A)

class Product < Dolly::Document
  property :sku, primary: true
end

Option B)

class Product < Dolly::Document
  property :sku

  primary :sku
end

p = Product.new
p.sku = "sau101"
p.id # "product/sau101"

It should work with either document propertys or arbitrary instance methods.

[javierg]

I prefere option "A", also, maybe explicitly override the "namespace" of the document which will sometimes differ from the class one, specially if we are using inheretance,

[seancookr]

This is an interesting feature to add. I shall ponder these ideas further

[seancookr]

I think this is also a feature and should not be included in 1.0 stable