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More about slave tables

We have been introduced to slave tables, but there is more to say about them.

Slave tables with special masters

The master of a slave table can be something else than a database row. For example

• in lino_xl.lib.accounting.MovementsByMatch the master is a str

• in lino_xl.lib.calview.DailyPlanner the master is a lino_xl.lib.calview.Day instance

• in lino_welfare.modlib.debts.PrintEntriesByBudget the master is an EntryGroup

If the master instance is not a subclass of lino.core.fields.TableRow, it must be JSON serializable.

Slave tables with remote master

The master_key of a slave table can be a remote field.

When you have three models A, B and C with A.b being a pointer to B and B.c being a pointer to C, then you can design a table CsByA which shows the C instances of a given A instance by saying:

class CsByA(Cs):
    master_key = "c__b"

For example lino_xl.lib.courses.ActivitiesByTopic shows all activities about a given topic. But an activity has no foreign key topic, so you cannot say master_key = 'topic'. But a course does know its topic indirectly because it knows its activity line, and the activity line knows its topic. So you can specify a remote field:

class ActivitiesByTopic(Courses):
    master_key = 'line__topic'

    allow_create = False

A slave table with a remote master should have allow_create set to False because we cannot set a line for a new course.

Other examples

• lino_avanti.lib.courses.RemindersByPupil

Slave tables with related master

Another special case is when you have the following structure where both orders and invoices are related to a partner, but the invoices don’t know their order.

The lino_xl.lib.orders.InvoicesByOrder table can be used in the detail of an order to show the invoices of the partner of that order. Here is how to define this case:

class InvoicesByOrder(InvoicesByPartner):

    label = _("Sales invoices (of client)")

    @classmethod
    def get_master_instance(cls, ar, model, pk):
        # the master instance of InvoicesByPartner must be a Partner, but since
        # we use this on an order, we get the pk of an order
        assert model is rt.models.contacts.Partner
        order = rt.models.orders.Order.objects.get(pk=pk)
        return order.get_invoiceable_partner()

Similar examples:

• lino_xl.lib.storage.MovementsByFiller

Multiple master fields

In Lino Prima, a cast is when a given teacher gives a given subject to a given group of pupils. And similarly, an exam is usually (but not always) done by a given teacher about a given subject in a given group.

But there is no direct pointer from exam to cast because not every exam requires a cast. When we open the detail view of a cast, we want to see a list of all exams for this cast. Here is how to declare the lino_prima.lib.ratings.ExamsByCast table:

class ExamsByCast(Exams):
    required_roles = dd.login_required(PrimaTeacher)
    master = "school.Cast"
    column_names = "seqno designation period *"

    @classmethod
    def get_request_queryset(self, ar, **filter):
        mi = ar.master_instance
        assert isinstance(mi, rt.models.school.Cast)
        qs = super().get_request_queryset(ar, **filter)
        # When there are two teachers for a same subject, they can optionally
        # manage exams together.
        if manage_exams_separately:
            qs = qs.filter(subject=mi.subject, group=mi.group, user=mi.user)
        else:
            qs = qs.filter(subject=mi.subject, group=mi.group)
        return qs