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Last active February 9, 2023 18:01
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Django Custom Model ForeignKey Field for Spanning Databases
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Sarrus1 commented Jul 17, 2020


Thanks you very much for your solution! However, it seems like migrations are not registering the SpanningForeignKey field. Adding them manually does not fix the issue aswell. Do you have any idea what is going wrong?


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@Sarrus1, whats the problem

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is there any solution for manytomanyfield?

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@Sarrus1, cut and paste the error for further help..@diegobill, have not seen one,but keep on looking, the internet is a strange place

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I'm trying to use this class to allow for a lookup of a user in auth_user in a 'default' DB , different from my application DB. ('test2' here).
the table I created looks like:

CREATE TABLE IF NOT EXISTS "test2_m1" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "description" text NOT NULL, "user_id" integer NOT NULL ); -- no foreign key

But when I run these statements, Django generates the following query wth an INNER JOIN to auth_user, and SpanningForeignKey.validate() is not called at all (I added a debug line to test):

from test2.models import *
M1.objects.get(user__username = 'xyz')
db_for_read: test2 ---> test2
ZZZZ SELECT "test2_m1"."id", "test2_m1"."user_id", "test2_m1"."description" FROM "test2_m1" INNER JOIN "auth_user" ON ("test2_m1"."user_id" = "auth_user"."id") WHERE "auth_user"."username" = xyz

The problem here is that the underlying Django code still wants to form a query with an INNER JOIN on the app-specific DB, not the default DB, expecting "auth_user" to be in that DB.

My questions:

  1. How do I get Django to NOT do that INNER JOIN on "auth_user" , especially when I specified no foreign key in the app's DB ? Shouldn't the SpanningForeignKey class prevent this from happening ?
  2. If SpanningForeignKey.validate() won't do that for me, what other method(s) must I override ?

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