Support for database links

[stephane.benchimol@…]

Currently, Oracle with Django do not support database links (see ​Oracle documentation)

It's used this way in SQL:

select count(*) from "MY_TABLE"@"MY_DBLINK";

With Django, I tried to passe it through the db_name

class MyObject(models.Model):
    class Meta:
        managed = False
        db_table = u'"MY_TABLE"@"MY_DBLINK"'

However, when it comes to write sql, Django use db_table to specify the column names. Using db link, the generated command is not valid:

SELECT "MY_TABLE"@"MY_DBLINK"."COLUMN1", "MY_TABLE"@"MY_DBLINK"."COLUMN2" FROM "MY_TABLE"@"MY_DBLINK"

It should be:

SELECT "MY_TABLE"."COLUMN1"@"MY_DBLINK", "MY_TABLE"."COLUMN2"@"MY_DBLINK" FROM "MY_TABLE"@"MY_DBLINK"

I think it worth to add a new db_link Meta option, eg.

class MyObject(models.Model):
    class Meta:
        managed = False
        db_table = u'MY_TABLE'
        db_link = u'MY_DBLINK'

A workaround could be to define a new database access in Django settings. However, I guess in some cases, the remote database could be reached only from the bridge - not directly from Django application. That's why this options should be needed.

[Russell Keith-Magee]

See #6148 for a vaguely related ticket. Based on the documentation provided, it also seems like there's crossover here with the feature request for cross-database joins (documented as not being supported in #13216)

As for the API approach, I'm not wild about the idea of adding a Meta option for an Oracle-specific feature. It feels to me like there should be some way to tie this to routing and the underlying database definition (i.e., the ​link is a feature of the database connection, not of the table itself).

[stephane.benchimol@…]

After few researches, it appears this function is not only Oracle-specific, but exists also in Postgresql (see ​Postgresql documentation) -- and maybe more databases ?

However, with Postgres, the approach is slightly different, because it has to encapsulate the query using dblink function, eg.

select * from dblink('dbname=MY_DBLINK', 'select count(*) from "MY_TABLE"')

[Anssi Kääriäinen]

I am currently working on #6148, and I think this could be relatively easy to support if the #6148 work gets completed. Just another syntax for the FROM entry.

If this gets implemented, db_link should imply managed = False.