Django ORM. Joining subquery - django

Question

I have a table which contains list of some web sites and a table with statistics of them.
class Site(models.Model):
domain_name = models.CharField(
max_length=256,
unique=True,
)
class Stats(models.Model):
date = models.DateField()
site = models.ForeignKey('Site')
google_pr = models.PositiveIntegerField()
class Meta:
unique_together = ('site', 'date')
I want to see all sites and statistics for a concrete date. If a stats record for the date doesn't exist, then the selection must contain only site.
If I use:
Site.objects.filter(stats__date=my_date)
I will not get sites which have no records for my_date in stats table. Because in this case the SQL query will be like the following:
SELECT *
FROM site
LEFT OUTER JOIN stats ON site.id = stats.site_id
WHERE stats.date = 'my_date'
The query condition will exclude records with NULL-dates and sites without stats will be not included to the selection.
In my case I need join stats table, which has already been filtered by date:
SELECT *
FROM site
LEFT OUTER JOIN
(SELECT *
FROM stats
WHERE stats.date = 'my-date') AS stats
ON site.id = stats.site_id
How can I translate this query to Django ORM?
Thanks.

Answer 1

In Django v2.0 use FilteredRelation
Site.objects.annotate(
t=FilteredRelation(
'stats', condition=Q(stats__date='my-date')
).filter(t__google_pr__in=[...])

Answer 2

I had a similar problem and wrote the following utility function for adding left outer join on a subqueryset using Django ORM.
The util is derived from a solution given to add custom left outer join to another table (not subquery) using Django ORM. Here is that solution: https://stackoverflow.com/a/37688104/2367394
Following is the util and all related code:
from django.db.models.fields.related import ForeignObject
from django.db.models.options import Options
from django.db.models.sql.where import ExtraWhere
from django.db.models.sql.datastructures import Join
class CustomJoin(Join):
def __init__(self, subquery, subquery_params, parent_alias, table_alias, join_type, join_field, nullable):
self.subquery_params = subquery_params
super(CustomJoin, self).__init__(subquery, parent_alias, table_alias, join_type, join_field, nullable)
def as_sql(self, compiler, connection):
"""
Generates the full
LEFT OUTER JOIN (somequery) alias ON alias.somecol = othertable.othercol, params
clause for this join.
"""
params = []
sql = []
alias_str = '' if self.table_alias == self.table_name else (' %s' % self.table_alias)
params.extend(self.subquery_params)
qn = compiler.quote_name_unless_alias
qn2 = connection.ops.quote_name
sql.append('%s (%s)%s ON (' % (self.join_type, self.table_name, alias_str))
for index, (lhs_col, rhs_col) in enumerate(self.join_cols):
if index != 0:
sql.append(' AND ')
sql.append('%s.%s = %s.%s' % (
qn(self.parent_alias),
qn2(lhs_col),
qn(self.table_alias),
qn2(rhs_col),
))
extra_cond = self.join_field.get_extra_restriction(
compiler.query.where_class, self.table_alias, self.parent_alias)
if extra_cond:
extra_sql, extra_params = compiler.compile(extra_cond)
extra_sql = 'AND (%s)' % extra_sql
params.extend(extra_params)
sql.append('%s' % extra_sql)
sql.append(')')
return ' '.join(sql), params
def join_to(table, subquery, table_field, subquery_field, queryset, alias):
"""
Add a join on `subquery` to `queryset` (having table `table`).
"""
# here you can set complex clause for join
def extra_join_cond(where_class, alias, related_alias):
if (alias, related_alias) == ('[sys].[columns]',
'[sys].[database_permissions]'):
where = '[sys].[columns].[column_id] = ' \
'[sys].[database_permissions].[minor_id]'
children = [ExtraWhere([where], ())]
return where_class(children)
return None
foreign_object = ForeignObject(to=subquery, from_fields=[None], to_fields=[None], rel=None)
foreign_object.opts = Options(table._meta)
foreign_object.opts.model = table
foreign_object.get_joining_columns = lambda: ((table_field, subquery_field),)
foreign_object.get_extra_restriction = extra_join_cond
subquery_sql, subquery_params = subquery.query.sql_with_params()
join = CustomJoin(
subquery_sql, subquery_params, table._meta.db_table,
alias, "LEFT JOIN", foreign_object, True)
queryset.query.join(join)
# hook for set alias
join.table_alias = alias
queryset.query.external_aliases.add(alias)
return queryset
join_to is the utility function you want to use. For your query you can use it in as follows:
sq = Stats.objects.filter(date=my_date)
q = Site.objects.filter()
q = join_to(Site, sq, 'id', 'site_id', q, 'stats')
And following statement would print a query similar to you example query (with subquery).
print q.query

Answer 3

Look at it this way: you want to see statistics with accompanying site data for certain date, which translates to:
Stats.objects.filter(date=my_date).select_related('site')