Mongoengine deferencing happens after using select_related()

问题

This could be a stupid question if my understanding about select_related() is completely wrong.

Here is the database design that I have.

class UserAccount(Document):
    first_name = StringField(max_length = 20)
    last_name = StringField(max_length = 20)
    user_name = StringField(max_length = 20) 
    friends = ListField(ReferenceField('self'))

And the queryset is:

u = UserAccount.objects.get(user_name="something").select_related()

I've passed the result in 'u' to the Django templates. So I tried this in the template.

{% for friend in u.friends %}
    {% for f in friend.friends %}
        {{ f.friends }}
    {% endfor %}
{% endfor %}

I'm able to see the friends of friends of my friends in the browser. Isn't the work of select_related() to stop the further dereferencing that's happening here?

Correct me if i'm wrong.

回答1:

First a word of warning, this is not only a relational schema but a recursively relational one. You could follow the relationships and end up loading all the UserAccount objects.

Remember there are no joins in MongoDB - so you are asking the client code (mongoengine) to perform in application joins for you - which means multiple queries and then assigning the results to the correct documents - thats expensive. So please ask yourself "is this the right tool for the job?"

However, your observations are correct - by default select_related() only goes down a single relation so for a user object it will only dereference the first set of friends and not their friends. You can change the depth by passing it to select_related(max_depth=2).

来源：https://stackoverflow.com/questions/20224141/mongoengine-deferencing-happens-after-using-select-related

标签

django

mongodb

mongoengine