Question

I need to get a list of all users for a given provider. A user may have multiple providers, and a provider may have multiple users. Here is what I have so far:

I have the following tables:

class Provider(models.Model):
    provider = models.CharField(max_length=100, primary_key=True)

class UserProfile(models.Model):
    user = models.ForeignKey(User, primary_key=True)
    provider = models.ManyToManyField(Provider, db_column='provider')

In my view:

providers = Provider.objects.order_by('provider')

And in my template:

{% for provider in providers %}
<tr>
    <td class="provider">
        {{ provider.provider }}
    </td>
    <td class="email">
        {% for profile in provider.userprofile_set.all %}
            {{ profile.user }}
        {% endfor %}    
    </td>
</tr>
{% endfor %}

This produces about 250 queries, as I’m iterating over the Provider QuerySet in the template. How would I improve the above — is there a way to use prefetch_related or select_related here?

Answer 1

Use prefetch_related(); select
_related() doesn’t work on ManyToManyFields:

providers = Provider.objects.order_by('provider').all().prefetch_related('userprofile_set')

From the docs:

Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.

This has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.

…

prefetch_related, on the other hand, does a separate lookup for each relationship, and does the ‘joining’ in Python. This allows it to prefetch many-to-many and many-to-one objects, which cannot be done using select_related, in addition to the foreign key and one-to-one relationships that are supported by select_related.