Use of Prefetch with an inner select_related in Django - django

Question

I basically have to display a list of service providers and in each, I need to display the categories of service they offer.
So as an example:
Possible Service Type Categories:
[id: 1, name:'Programming']
[id: 2, name:'Design']
Possible Service Types:
[id: 1, name: 'PHP Service', service_type_category_id: 1]
[id: 2, name: 'JAVA Service', service_type_category_id: 1]
[id: 3, name: 'Web Design Service', service_type_category_id: 2]
Example of Display Results:
Company Blue offers 'Programming'
Company Test offers 'Programming' and 'Design'
Company Orange offers 'Design' ....
I'm trying to write the least number of queries:
I have these models:
class ServiceTypeCategory( BaseModel ):
# Model Attributes
name = models.CharField( _( "name" ), max_length = 40 )
class ServiceType( BaseModel ):
# Model Attributes
service_type_category = models.ForeignKey( 'ServiceTypeCategory', verbose_name = _( 'category' ) )
name = models.CharField( _( "name" ), max_length = 60 )
description = models.TextField( _( "description" ) )
class Provider( BaseModel ):
# Model Attributes
display_name = models.CharField( _( "name" ), max_length = 80 )
# Many to many relations
countries = models.ManyToManyField( 'core.Country' ) # countries this provider support
service_types = models.ManyToManyField( 'ServiceType', through = 'Provider_ServiceTypes', related_name = 'service_types' )
class Provider_ServiceTypes( BaseModel ):
# Model Attributes
service_type = models.ForeignKey( 'ServiceType', verbose_name = _( 'service type' ) )
provider = models.ForeignKey( 'Provider', verbose_name = _( 'provider' ) )
is_top = models.BooleanField( _( "is top service" ), default = False )
Then, to run the query, I have the following:
providers = Provider.objects.select_related(
'user',
).prefetch_related(
Prefetch(
'service_types__service_type_category',
queryset = ServiceTypeCategory.objects
.only( 'name' )
)
).filter(
countries = country_id,
).only(
'id', 'display_name', 'user'
).order_by(
'-user__last_login'
)
This works out well, but it runs the 3 following queries:
SELECT app_provider.id, app_provider.user_id, app_provider.display_name, core_user.id, core_user.password, core_user.last_login, core_user.is_superuser, core_user.created_date, core_user.modified_date, core_user.email, core_user.name, core_user.is_active, core_user.is_admin
FROM app_provider
INNER JOIN app_provider_countries ON ( app_provider.id = app_provider_countries.provider_id )
INNER JOIN core_user ON ( app_provider.user_id = core_user.id )
LEFT OUTER JOIN core_userpersonal ON ( core_user.id = core_userpersonal.user_id )
LEFT OUTER JOIN core_userstats ON ( core_user.id = core_userstats.user_id )
WHERE app_provider_countries.country_id = 204
ORDER BY core_userstats.total_reviews DESC, core_userstats.total_contracts DESC, core_userstats.total_answers DESC, core_user.last_login DESC LIMIT 5
SELECT (app_provider_servicetypes.provider_id) AS _prefetch_related_val_provider_id, app_servicetype.id, app_servicetype.created_date, app_servicetype.modified_date, app_servicetype.service_type_category_id, app_servicetype.name, app_servicetype.description
FROM app_servicetype
INNER JOIN app_provider_servicetypes ON ( app_servicetype.id = app_provider_servicetypes.service_type_id )
WHERE app_provider_servicetypes.provider_id IN (2)
SELECT app_servicetypecategory.id, app_servicetypecategory.name
FROM app_servicetypecategory
WHERE app_servicetypecategory.id IN (1, 2)
Question is: How can I make to run just 2 queries in total? (The last 2 queries should be joined with INNER JOIN and a group by per service_type_category_name)
Thanks in advance!

Answer 1

Try this:
providers = Provider.objects.select_related(
'user',
).prefetch_related(
Prefetch(
'service_types',
queryset = ServiceType.objects\
.select_related('service_type_category')\
.only( 'service_type_category', 'name' )
)
).filter(
countries = country_id,
).only(
'id', 'display_name', 'user'
).order_by(
'-user__last_login'
)