If you want to update a single document by hand there is a gotcha. When you run a find it looks like you are getting one result but you are actually getting a set of records which contains 1 entry.

Log into mongo:

> var x = db.person.find({"fake_dir":"/mrn/dir/thing"});
> var y = x.next();
> print(y);
[object BSON]
> y.XXXX = "blar blar";
blar blar
> y.   // double tab to print the commands available
y.XXXX                   y.constructor            y.name                   y.toLocaleString(
y._id                    y.created_at             y.names                  y.toString(
y.bsonsize(              y.fake_dir               y.owner_id               y.updated_at
y.clone_id               y.hasOwnProperty(        y.propertyIsEnumerable(  y.valueOf(
> db.person.save(y);
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })