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How to define a learning foreign key

The application developer can turn any foreign key field into a learning foreign key.

Learning Comboboxes

When the model also defines a method create_<fieldname>_choice, then the chooser will become "learning": the ComboBox will be told to accept also new values, and the server will handle these cases accordingly.

In the example application you can create new cities by simply typing them into the combobox.

class Person(dd.Model):
    ...
    def create_city_choice(self, text):
        """
        Called when an unknown city name was given.
        Try to auto-create it.
        """
        if self.country is not None:
            return rt.models.countries.Place.lookup_or_create(
                'name', text, country=self.country)

        raise ValidationError(
            "Cannot auto-create city %r if country is empty", text)

Usage

• Define a chooser for the field by defining a FOO_choices() method on the model and decorating it with @dd.chooser.

learning chooser

The chooser associated to a learning foreign key.

• Define an instance method create_FOO_choice on the model.

When you use the lino.core.model.Model.create_from_choice() method, you probably want to override the model's lino.core.model.Model.choice_text_to_dict method of the related model.

Examples

The city field of a postal address. When you specify the name of a city that does not yet exist, you simply leave the "invalid" city name in the combobox (Lino accepts this on a learning foreign key) and save the partner. Lino will then silently create a city of that name. Note that you must at least set the country, otherwise Lino refuses to automatically create a city. This behaviour is defined in the countries.CountryCity model mixin, which is inherited e.g. by contacts.Partner. or addresses.Address

Example from lino_xl.lib.countries.CountryCity:

def create_city_choice(self, text):
    if self.country is not None:
        return rt.models.countries.Place.lookup_or_create(
            'name', text, country=self.country)

    raise ValidationError(
        "Cannot auto-create city %r if country is empty", text)

Or the lino_xl.lib.contacts.Role.person field. You can see the feature in every application with contacts. For example lino_book.projects.min1. In the detail of a company, you have the RolesByCompany slave table. In the Person column of that table you can type the name of a person that does not yet exist in the database. Lino will create it silently, and you can then click on the pointer to edit more information.

Example from lino_xl.lib.contacts.Person:

def create_person_choice(self, text):
    return rt.models.contacts.Person.create_from_choice(text)

See also doctest snippets in Automatically creating contact persons.

Implementation details

When a method is decorated with the chooser decorator, Lino creates a lino.utils.choosers.Chooser instance. The can_create_choice attribute of this instance will automatically be set to True when the field's model also has a method named create_FOO_choice (FOO being the field name).