Specify the database for a Django ModelForm instance

Question

How can I specify which database (by its alias name) a Django ModelForm should use?

A Django ModelForm knows its corresponding model, and the fields included.

The ModelForm instance clearly knows how to specify a database, internally. It can validate its fields against the database, and can save a new model instance to the database. This implies its operations have knowledge of which database to use.

I can't find how to specify any database other than the default, when creating the ModelForm nor when it interacts with the database::

import csv

from cumquat_app.forms import CumquatImportForm

db_alias = 'foo'

reader = csv.DictReader(input_file)
for row in reader:
    fields = make_fields_from_input_row(reader)

    # Wanted: ‘form = CumquatInputForm(fields, using=db_alias)’.
    form = CumquatImportForm(fields)

    # Wanted: ‘if form.is_valid(using=db_alias)’.
    if form.is_valid():

        # Wanted: ‘form.save(using=db_alias)’.
        form.save()

What I need is to specify the database alias as an external user of the ModelForm, when creating the instance or when calling ModelForm.clean or ModelForm.is_valid or ModelForm.save etc.

The same way I can with the ‘using’ hook of QuerySet.using('foo'), or Model.save(using='foo').

• Note that this is not a job for multi-database routing policy configuration. The use case is that I need to specify exactly one database, only known at run time. If the connection fails it should not fall back to any other, so database routes are the wrong hammer for this nail.

• I can request the ModelForm.save method to not commit its change (with commit=False) and then use the Model.save directly. That does not address the other behaviour of a ModelForm which accesses the database, so it is not a solution to this question.

• A ModelManager.db_manager could do the job, if I use it to create the model instance. But I'm relying on the form to create the instance; I can't create a model instance because I don't have field values to assign yet. That's the job of the form.

• If it matters: this is in a management command, where I need to be able to specify from the command line that a particular database alias is the context for a command.

What is the equivalent for using='foo' when instantiating a ModelForm for the model, or calling its methods (ModelForm.clean, ModelForm.save, etc.)?

Answer 1

Unless someone can find a way to do it as requested using the ModelForm interface, I can only conclude Django offers no way to do this with the ModelForm API as it stands.