So many times in working with trees I have a model, say Group, that has an associated tree, say of Users (say, with parent/child relationship defined by referral, as in the closure_tree specs). I keep a reference to the root of the tree, and would like to be able to load the root and all its descendants. I know that self_and_descendants can do this, but I want two things in addition:

1. I don't want an array of nodes--I still want to be able to traverse the eager-loaded nodes using the familar .parent and .children methods. This is the way you're supposed to traverse a tree and this is how tree recursion typically and elegantly works.
2. I still want to be able to eager load associations of the User class.

Unless I'm missing something, there is no way to do this currently. self_and_descendants gives you #2 but not #1.

This PR allows both. The basic idea is you declare:

has_closure_tree_root :root_user

This defines a plain has_one association to the root node (no preloading here), but also lets you do:

some_group.root_user_including_tree(contracts: :contract_type)

which loads all Users, Contracts, and ContractTypes in 4 queries, no matter how many nodes are in the tree. .parent and .children work as expected. Once the root is loaded, something like root.children[0].children[0].contracts[0].user.parent.parent.children[1].children[1].contracts[0].contract_type.name triggers zero queries (not that you would ever write this line directly, but you get the idea).

It also preloads the inverse belongs_to :group association on User, if that association is defined. So root.children[0].group also triggers zero queries.

I'm curious for your thoughts on this. It's been very useful in my projects and curious if you think it would be a useful addition.

And thanks for the always awesome closure_tree!