Question

I have a data structure that I’m looking to model and persist in rails. It’s self-referential and would look a lot like a hierarchical tree however instead of having just one parent each node could have multiple parents and multiple children.

It might look something like this (all nodes are the same model):

                       Foo            Bonk
                     |    |          |    |
                    Bar   Baz      Foo   Bork 
                   |   |
                 Baz   Bork

Where all of the nodes with the same name are actually the same node.

I also want to find all instances of the model that contain a certain “ingredient.”

The closest thing I found was https://github.com/mceachen/closure_tree but as far as I can tell each node only has one parent.

I’m ideally looking for a pattern to implement with ActiveRecord, but will consider any solution.

Answer 1

For each node having only one parent, this is a good way to do it:

has_many :children, :class_name => "Node", :foreign_key => "parent_id"
belongs_to :parent, :class_name => "Node", :foreign_key => "parent_id"

As you can tell, its an ActiveRecord model with a field named parent_id pointing to the parent node.

For multiple parents, this can be extended to create a many-to-many self referential join.

has_and_belongs_to_many :children, :class_name => "Node",
    :foreign_key => "node_id",
    :association_foreign_key => "children_id",
    :join_table => "children_nodes"

Create the join table, now you should be able to do Node.children and Node.nodes for parents.
Note: The second part has not been tested, but let me know if it doesn’t work.