LEFT JOIN Django ORM

Question

I have the following models:

class Volunteer(models.Model):
    first_name = models.CharField(max_length=50L)
    last_name = models.CharField(max_length=50L         


        

Answer 1

To me were need custom join models, that have implicit fields
it work to me on django 1.9.
but it more seem on the crutch
If someone have more elegant solution please share for people

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

result:

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

here example of use with additional condition and set table alias(but it seem as crutch)

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

i use it by

# 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'll be

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)