Question

I have a Mongodb collection and I have created a Python class for documents in the collection. The class has some properties and methods which are not stored with the document. Should I try and store them to make the properties searchable or should I not store them and search the objects in Python?

Here is an example:

# Child
class Child:
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    @property
    def parent(self):
        try:
            return Parent(**db.Parents.find_one({'child':self._id}))
        except:
            return None

# Parent
class Parent:
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    @property
    def child(self):
        try:
            return Child(**db.Children.find_one({'parent':self._id}))
        except:
            return None

In this example, to search for all the children who’s parent’s name is “foo”, I have to do this:

results = [Child(**c) for c in db.Children.find() if c.parent.name == 'foo']

This means I have to pull all the Children documents from Mongodb and search them. Is it smarter to write the Parent data (or a subset of it) to the Child document, so I can use Mongodb to do the searching?? So my Child class could look like this:

# Child
class Child:
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    @property
    def parent_name(self):
        try:
            return db.Parents.find_one({'child':self._id})['name']
        except:
            return None

    def _save(self):
        # something like this to get and save all the properties
        data = {m[0]:getattr(self,m[0]) for m in inspect.getmembers(self)}
        db.Children.find_and_modify({'_id':self._id},{'$set':data},upsert=True)

# search
results = [Child(**c) for c in db.Children.find({'parent_name':'foo'})]

So the search is more efficient, but I think having to keep the Child objects updated could be painful and dangerous. If I change the name of a Parent, I have to also rewrite its children. Feels wrong. Any better ideas???

Answer 1

You don’t have to load all Children.

parent_ids = db.Parents.find({'name': 'foo'}).distinct('_id')
children = db.Children.find({'parent': {'$in': parent_ids}})

(Also, why do you have both a child field on a parent and a parent field on a child?)