How can I query ancestral relationships in MongoDB? - mongodb

Question

I'm reading the documentation for tree structures here:
http://docs.mongodb.org/manual/tutorial/model-tree-structures/
The hierarchy is:
Books -> Programming -> [Languages, Databases -> [Postgres, MongoDB]]
In the documentation it says:
The query to retrieve the parent of a node is fast and straightforward:
db.categories.findOne( { _id: "MongoDB" } ).parent
That makes sense. However, how can I run a query based on attributes of the ancestor? For example, instead of just retrieving the parent, suppose I wish to find all documents where the grandfather has an _id of "Books" how would I do that? The answer should be "Languages" and "Databases".

Answer 1

If your tree structure is fixed. You can query by following way, before that, just change your MongoDB structure by following URL,
http://docs.mongodb.org/manual/tutorial/model-tree-structures-with-nested-sets/
to get all ancestor of Databases,
var databaseCategory = db.categories.findOne( { _id: "Databases" } );
db.categories.find( { left: { $lt: databaseCategory.left }, right: { $gt: databaseCategory.right } } );
Result:
{ "_id" : "Books", "parent" : 0, "left" : 1, "right" : 12 }
{ "_id" : "Programming", "parent" : "Books", "left" : 2, "right" : 11 }
to get all descendant of Databases,
var databaseCategory = db.categories.findOne( { _id: "Databases" } );
db.categories.find( { left: { $gt: databaseCategory.left }, right: { $lt: databaseCategory.right } } );
Result:
{ "_id" : "MongoDB", "parent" : "Databases", "left" : 6, "right" : 7 }
{ "_id" : "dbm", "parent" : "Databases", "left" : 8, "right" : 9 }