Allow updating fields using expressions in bulk_create with update_conflicts

[Take Weiland]

My use-case is as follows:
I have a list of objects that I need to "upsert" into the database. However one of the fields must only be updated if it has no value yet.
In raw SQL (PostgreSQL) I would write this as follows:

INSERT INTO my_table (id, user_id, field_a, field_b)
VALUES (...), (...), (...)
ON CONFLICT (user_id) DO UPDATE SET field_a = EXCLUDED.field_a, field_b = COALESCE(my_table.field_b, EXCLUDED.field_b)
RETURNING id;

There is no way to express this query using Django's ORM as far as I can tell.

INSERT ... ON CONFLICT is covered by bulk_create with update_conflicts. However it only lets you specify a list of fields to update.

Some API bike-shedding: I could imagine it being supported like this:

MyModel.objects.bulk_create(
  objs,
  update_conflicts=True,
  update_fields={
    'field_b': Coalesce(F('field_b'), Inserted('field_b'))
  }
)

Note the use of the pseudo DB-Function Inserted. It allows refering to the values that would originally be inserted. There is different syntax for this depending on the DB Vendor:

• PostgreSQL and SQLite use the pseudo-table-name "excluded": excluded.field_name (​https://www.postgresql.org/docs/current/sql-insert.html, ​https://sqlite.org/lang_upsert.html)
• MariaDB uses the VALUE-function: VALUE(field_name) (​https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/inserting-loading-data/insert-on-duplicate-key-update, ​https://mariadb.com/docs/server/reference/sql-functions/secondary-functions/miscellaneous-functions/values-value)
• MySQL uses the pseudo-table-name "new": new.field_name (​https://dev.mysql.com/doc/refman/9.4/en/insert-on-duplicate.html)
• Oracle doesn't support ON DUPLICATE KEY UPDATE or a similar construct (this is already a limitation of update_conflicts).

This functionality is already implemented in the database operations in on_conflict_suffix_sql, however it is hardcoded to not allow custom expressions.

[Take Weiland]

See https://code.djangoproject.com/ticket/35793