Database Reference
In-Depth Information
Figure 5-3. Adding Swing to the hierarchy
Now we need to update the ancestor list of the
bop
document
and all its descendants
. In order
to do this, we'll first build the subgraph of
bop
in memory, including all of the descendants of
bop
, and then calculate and store the ancestor list for each node in the subgraph.
For the purposes of calculating the ancestor list, we will store the subgraph in a
dict
contain-
ing all the nodes in the subgraph, keyed by their
parent
field. This will allow us to quickly
traverse the hierarchy, starting with the
bop
node and visiting the nodes in order:
from
from
collections
collections
import
import
defaultdict
def
def
build_subgraph
(
root
):
nodes
=
db
.
categories
.
find
(
{
'ancestors._id'
:
root
[
'_id'
] },
{
'parent'
:
1
,
'name'
:
1
,
'slug'
:
1
,
'ancestors'
:
1
})
nodes_by_parent
=
defaultdict
(
list
)
for
for
n
iin
nodes
:
nodes_py_parent
[
n
[
'parent'
]]
.
append
(
n
)
return
return
nodes_by_parent
The
defaultdict
from the Python standard library is a dictionary with a special
behavior when you try to access a key that is not there. In this case, rather than rais-
ing a
KeyError
like a regular
dict
, it will generate a new value based on a factory
