How to add queryset methods to a measurement type#

A queryset method gives a query you repeat a name, such as “the laser diffraction results”. You add one to a measurement type’s manager, and it chains with the filters and loaders that every measurement queryset has.

Before you start#

You need a measurement type. This guide uses GrainSizeMeasurement from How to add a measurement type.

Steps#

Subclass MeasurementQuerySet and add your methods. Each method returns a queryset.

# sediments/models.py
from fairdm.core.measurement.managers import MeasurementQuerySet


class GrainSizeQuerySet(MeasurementQuerySet):
    def by_method(self, method):
        return self.filter(method__iexact=method)

    def with_uncertainty(self):
        return self.exclude(uncertainty__isnull=True)

Subclassing MeasurementQuerySet, not QuerySet, keeps with_related(), with_metadata(), the location lookups and the visibility methods. They are listed on Measurement model.

Attach the queryset to the model as its manager. Define the queryset above the model.
```
class GrainSizeMeasurement(Measurement):
    # ... fields ...

    objects = GrainSizeQuerySet.as_manager()
```
FairDM turns the manager into a polymorphic one, so a query on the type still returns the right subclass.

Chain your methods with the built-in ones.

GrainSizeMeasurement.objects.by_method("laser diffraction").with_uncertainty().with_related()

The result#

GrainSizeMeasurement.objects has your methods and the methods of MeasurementQuerySet. A queryset method adds no column, so it needs no migration.

Load related records with with_related() on a list, so each row does not cost extra queries. Use with_metadata() where the page shows descriptions, dates or identifiers.