Producers

Producer functions take a model instance as their single argument and return a value derived from that instance. The producers module contains higher-order functions that create producers.

Import like this: from django_readers import producers

producers.attr(name, *, transform_value=None, transform_value_if_none=False)

Returns a producer which gets the specified attribute name from the instance. Most useful for retrieving the value of model fields.

instance = Book(title="Pro Django")
produce_title = producers.attr("title")
value = produce_title(instance)
print(value)
# prints "Pro Django"

The producers.attr function takes an optional argument transform_value, which is a function that receives the value of the attribute and returns a new value. This is useful if the value of the attribute needs to be converted in some way.

For example, imagine you have an IntegerField but you want the produced value to be a stringified version of the integer value. In that case, you can use producers.attr("my_integer_field", transform_value=str).

By default, the transform_value function is only called if the value of the attribute is not None (so in the example above, if the database value of my_integer_field is NULL then None would be returned, rather than the string "None"). If you want the transform_value function to always be called, use producers.attr("my_integer_field", transform_value=str, transform_value_if_none=True).

producers.method(name, /, *args, **kwargs)

Returns a producer that calls the method name on the instance. If additional arguments and/or keyword arguments are given, they will be given to the method as well. For example:

class Book(models.Model):
    ...

    def published_in_year(self, year):
        return self.publication_date.year == year


instance = Book(publication_year=2020)
produce_published_in_2020 = producers.method("published_in_year", 2020)
value = produce_published_in_2020(instance)
print(value)
# prints "True"

producers.relationship(name, related_projector)

Note

You shouldn't generally need to use this function directly, instead it would be called for you via a higher-level construct such as a pair function or a spec.

Given an attribute name and a projector, return a producer which plucks the attribute off the instance, figures out whether it represents a single object or an iterable/queryset of objects, and applies the given projector to the related object or objects.

producers.pk_list(name)

Note

You shouldn't generally need to use this function directly, instead it would be called for you via a higher-level pair function.

Given an attribute name (which should be a relationship field), return a producer which returns a flat list of the primary key of each item in the relationship (or just a single PK if this is a to-one field, but this is usually an inefficient way of doing it).