Design concepts & constraints
The basic concept of using real numbers to implement the nested set model in reltational databases is quite simple. But as always the devil is in the detail. SQL is a set based language and it works quite well as a declarative language, but Ruby has a definite imperative flavour and that means you have to worry about doing things in the right order. Also although we're using real numbers computers don't actually work like that, they emulate real numbers up to a finite precision by using binary numbers. That means that we do have to worry about running out of gaps between any two numbers. We don't run into that problem straight away, like we'd do if using integers for nested sets but we will run into the problem at some point.
That means when we put an item inside a parent we have to be careful to leave a good amount of space to put more elements inside the parent without running out of real numbers. For example we could have a parent node with lft = 1 and rgt = 2, we put one child inside it with lft=rgt=1.5. The next child we'd put at lft=rgt=1.75, the next at 1.875, i.e at half the gap between the largest child rgt and the parent rgt = 2. It's very uneven, and we could go back a reorganise our numbering each time we add a child, but that's the very thing we want to avoid doing because that's the whole point of using real numbers instead of integers, you shouldn't have to juggle things about to make space for new entries. If we were using .5 of the gap left then after a million children we'd be beyond our finite precision limit. The way I've approached this is to assume that most of the time trees are wide and shallow and therefore we can maintain space be putting new items in say 1/1E6 of the space left. That way we'll keep a lot of space available for new siblings, but we might run out of space if we try to build very deep trees. I'll leave it as an excerise for the reader to experiment with this and find out where the limits lie. You can adjust the amount of available space each item uses up by setting the bias, the default is 1E6
Going back to the issue of imperative vs declarative styles. We have to think about records in their different states. While the model is a collection of records in a relational database we can use SQL to act declaratively on whole sets at a time. The relation ships are determined only by the relations between the lfts and the rgts. But when the records are loaded into Ruby as ActiveRecord instances we give them a different set of relationships based on things like “parent” and “children” and we maintain variables to keep the tree intact. Loading and saving the records means translating between a