Add from_db_value() support to django.contrib.postgres.fields.ArrayField.

[Karan Lyons]

Currently ArrayField assumes that the base field will not need to alter the returned python value from the postgres backend, as this is the case for all core fields. As an example, take this absurd field:

class ContrivedStringedIntegerField(models.IntegerField):
	def from_db_value(self, value, expression, connection, context):
		return unicode(value)

Used alone in a model this will return a string, but used as part of an ArrayField we’ll get integers instead.

from_db_value() is a no-op for performance reasons, so rather than incur the penalty inside of ArrayField when it’s unnecessary, I propose the attached patch, which adds a from_db_value() method to the instantiated ArrayField if its base field has one as well.

[Karan Lyons]

Patch: Conditionally add from_db_value() to ArrayField instances.

[Simon Charette]

Makes sense to me but this will need additional tests to get merged.

If you could submit your patch as Github PR it would also give it more visibility and help reviewers run CI against it.

Concerning the implementation I think you should avoid creating a local function to attach it as a method but declare a _from_db_value method that you assign to from_db_value instead.

[Tim Graham]

Duplicate of #25143