左加入 Django ORM
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【中文标题】左加入 Django ORM【英文标题】：LEFT JOIN Django ORM 【发布时间】：2014-02-11 20:32:46 【问题描述】：我有以下型号：
class Volunteer(models.Model):
first_name = models.CharField(max_length=50L)
last_name = models.CharField(max_length=50L)
email = models.CharField(max_length=50L)
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
class Department(models.Model):
name = models.CharField(max_length=50L, unique=True)
overseer = models.ForeignKey(Volunteer, blank=True, null=True)
location = models.CharField(max_length=100L, null=True)
class DepartmentVolunteer(models.Model):
volunteer = models.ForeignKey(Volunteer)
department = models.ForeignKey(Department)
assistant = models.BooleanField(default=False)
keyman = models.BooleanField(default=False)
captain = models.BooleanField(default=False)
location = models.CharField(max_length=100L, blank=True, null=True)
我想查询所有没有分配志愿者的部门。我可以使用以下查询来做到这一点：
SELECT
d.name
FROM
vsp_department AS d
LEFT JOIN vsp_departmentvolunteer AS dv
ON d.id = dv.department_id
WHERE
dv.department_id IS NULL;
是否有更类似于 django 的方式来执行此操作，或者我应该只使用原始 sql 吗？
【问题讨论】：
【参考方案1】：您可以通过遵循查找中的向后关系来做到这一点。
>>> qs = Department.objects.filter(departmentvolunteer__isnull=True).values_list('name', flat=True)
>>> print(qs.query)
SELECT "app_department"."name" FROM "app_department" LEFT OUTER JOIN
"app_departmentvolunteer" ON ( "app_department"."id" = "app_departmentvolunteer"."department_id" )
WHERE "app_epartmentvolunteer"."id" IS NULL
这里是关于“跨越多值关系”查询的文档：https://docs.djangoproject.com/en/stable/topics/db/queries/#spanning-multi-valued-relationships
【讨论】：
谢谢！我会将您的解决方案与我的解决方案进行比较。 您的查询是否检查了 DepartmentVolunteer 中的 id 字段？还是 Django 会建立关系并检查 department_id 字段？ 它正在检查DepartmentVolunteer 中的 department 列，该列是 Departement 的 FK，因此匹配其 ID。
这是一个外连接。如果您不希望那里也有正确的连接匹配怎么办...？
我的查询是INNER JOIN。我正在使用django 1.11.20。关于如何进行左连接的任何想法？我需要一个INNER JOIN 和一些LEFT JOIN。我一直在尝试搜索它，但无法找到解决方案。【参考方案2】：
对我来说，需要具有隐式字段的自定义连接模型 它在 django 1.9 上对我有用。 但它更像是在拐杖上 如果有人有更优雅的解决方案，请分享给人们
from django.db.models.sql.datastructures import Join
from django.db.models.fields.related import ForeignObject
from django.db.models.options import Options
from myapp.models import Ace
from myapp.models import Subject
jf = ForeignObject(
to=Subject,
on_delete=lambda: x,
from_fields=[None],
to_fields=[None],
rel=None,
related_name=None
)
jf.opts = Options(Ace._meta)
jf.opts.model = Ace
jf.get_joining_columns = lambda: (("subj", "name"),)
j=Join(
Subject._meta.db_table, Ace._meta.db_table,
'T1', "LEFT JOIN", jf, True)
q=Ace.objects.filter(version=296)
q.query.join(j)
print q.query
结果：
SELECT
`ace`.`id`,
`ace`.`version_id`,
`ace`.`obj`,
`ace`.`subj`,
`ace`.`ACE_Type`,
`ace`.`ACE_Inheritance`,
`ace`.`ACE_Rights`
FROM `ace`
LEFT OUTER JOIN `core_subject`
ON (`ace`.`subj` = `core_subject`.`name`)
WHERE `ace`.`version_id` = 296
此处使用附加条件和设置表别名的示例（但它看起来像拐杖）
def join_to(self, table1, table2, field1, field2, queryset, alias=''):
"""
table1 base
"""
# 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], ())]
wh = where_class(children)
return wh
return None
dpj = ForeignObject(
to=table2,
on_delete=lambda: None,
from_fields=[None],
to_fields=[None],
rel=None,
related_name=None
)
dpj.opts = Options(table1._meta)
dpj.opts.model = table1
dpj.get_joining_columns = lambda: ((field1, field2),)
dpj.get_extra_restriction = extra_join_cond
dj = Join(
table2._meta.db_table, table1._meta.db_table,
'T', "LEFT JOIN", dpj, True)
ac = queryset._clone()
ac.query.join(dj)
# hook for set alias
alias and setattr(dj, 'table_alias', alias)
return ac
我用它
# how it use:
from django.db.models.expressions import Col
q = Something.objects \
.filter(type__in=["'S'", "'U'", "'G'"]) \
.exclude(name__in=("'sys'", "'INFORMATION_SCHEMA'")) \
.annotate(
... some annotation fields
class_= Col(Permissions._meta.db_table,
Permissions._meta.get_field('field_name'),
output_field=IntegerField()),
Grant=Col(
'T10',
Principals._meta.get_field('name'),
output_field=CharField()),
).values('Grant')
ac = self.join_to(Principals, ServerPrincipals, 'sid', 'sid', q)
# here invoke "extra_join_cond" of function "join_to"
ac = self.join_to(Permissions, Columns, 'major_id', 'object_id', ac)
# here use alias table
ac = self.join_to(Permissions, Principals, 'grantor_id', 'principal_id', ac, 'T10') # T10 is alias
sql 会是
SELECT
T10.name AS Grant
FROM sys.principals
LEFT OUTER JOIN sys.server_principals
ON (sys.principals.sid = sys.server_principals.sid)
LEFT OUTER JOIN sys.columns
ON (sys.permissions.major_id = sys.columns.object_id
AND (
(sys.columns.column_id = sys.permissions.minor_id))
)
LEFT OUTER JOIN sys.principals T10
ON (sys.permissions.grantor_id = T10.principal_id)
【讨论】：
太棒了！先生，你让我很开心。为了添加一点补充，我已经用我的查询创建了 Join，所以我的解决方案是覆盖它;)queryset.query.alias_map['my_outer_table'].join_field.get_extra_restriction = extra_join_cond
谢谢@madjardi。您的回答帮助我解决了这个问题：***.com/a/42816689/2367394【参考方案3】：
这似乎有效：
Department.objects.filter(departmentvolunteer__department__isnull=True)
更多详情请见docs。
【讨论】：
【参考方案4】：通过OR
创建自定义连接def get_queryset(self):
qs = super(AceViewSet, self).get_queryset()
qs = qs.select_related('xxx')
# construct all tables and the join dependence
qs.query.__str__()
qs.query.alias_map['xx_subject'].join_cols = (('xxx_id', 'uid'), ('xxx_id', 'ad_subject_id'))
qs.query.alias_map['xx_subject'].as_sql = partial(self.as_sql, qs.query.alias_map['xx_subject'])
return qs
@staticmethod
def as_sql(self, compiler, connection):
sql, params = Join.as_sql(self, compiler, connection)
or_sql = sql.replace("AND", "OR")
return or_sql, params
FROM "xx_ace"
LEFT OUTER JOIN "xx_subject"
ON ("xx_ace"."xxx_id" = "xx_subject"."uid" OR "xx_ace"."xxx_id" = "xx_subject"."ad_subject_id")
【讨论】：
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