Hello,i'll describe problem in the detail.The main problem is that i can't save 'res_model' field in 'ir.attachment' table. I use odoo v12.0.

Budget model​

```

class AccountBudget(models.Model):
    _name = "account.budget"

    attachment_ids = fields.One2many('ir.attachment', 'id', string='Attachments')
```

​

Budget view

```

<field name="attachment_ids" context="{'default_res_model': 'account.budget',
                                       'default_file_type': 'document'}">

</field>

```

As you can see, i set context wit default res_model. But when i click save button it doesn't save res_model.
File type work greatly.

This behavior only with One2Many relation. If i set 'attachment_ids' Many2Many all works fine.
But with Many2Many field i have one layer, that displayed all files, i don't need that.

How can i save attachment for certain res_model?