Cascade Soft Delete

[Steven Stelmach]

Hi all,

I'm working with models (for a newspaper site) similar to this:

- Abstract class Content with child class Article.
- Each Content object is related one-to-one with a ContentGeneric,
which has a published_status field.
- Articles have related_content, which is a M2M field with
ContentGeneric.

I've implemented a soft delete on ContentGeneric by overriding the
get_queryset method on my manager to only fetch content with active
published_statuses, but when I ask an article for its related_content
via the ORM, references to soft deleted content are obviously still
valid and are returned.

My question is basically: is there a way to make a default m2m manager
that fetches only active content? I'd like to be able to do something
like Articles.related_content.all() and have it return only active
related_content content instances.

Thanks,
Steve

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[Russell Keith-Magee]

Post by Steven Stelmach
Hi all,
- Abstract class Content with child class Article.
- Each Content object is related one-to-one with a ContentGeneric,
which has a published_status field.
- Articles have related_content, which is a M2M field with
ContentGeneric.
I've implemented a soft delete on ContentGeneric by overriding the
get_queryset method on my manager to only fetch content with active
published_statuses, but when I ask an article for its related_content
via the ORM, references to soft deleted content are obviously still
valid and are returned.
My question is basically: is there a way to make a default m2m manager
that fetches only active content? I'd like to be able to do something
like Articles.related_content.all() and have it return only active
related_content content instances.

You can do this by setting the default manager on the related table,
but this will affect all queries on the related table, not just the
m2m queries.

When you do an m2m query, Django uses the default manager on the
related model to construct the query - that is, if A and B have an m2m
relation, a query on A.b_set.all() uses B's default manager to
retrieve results. If B's default manager only returns active content,
then so will the m2m query.

However, this then means that the default manager on B will only
return active content, so explicit requests on B will be filtered by
default.

There isn't any way to specify the manager to use only in an m2m
relation - either as a model configuration item or a query parameter.
Adding this capability is something that has been proposed in the
past; to my recollection, the sticking point has been finding an
elegant way to express this configuration.

Yours,
Russ Magee %-)

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[Russell Keith-Magee]

Post by Steven Stelmach
Hi all,
- Abstract class Content with child class Article.
- Each Content object is related one-to-one with a ContentGeneric,
which has a published_status field.
- Articles have related_content, which is a M2M field with
ContentGeneric.
I've implemented a soft delete on ContentGeneric by overriding the
get_queryset method on my manager to only fetch content with active
published_statuses, but when I ask an article for its related_content
via the ORM, references to soft deleted content are obviously still
valid and are returned.
My question is basically: is there a way to make a default m2m manager
that fetches only active content? I'd like to be able to do something
like Articles.related_content.all() and have it return only active
related_content content instances.

... and just after I posted, my brain kicked into gear and thought of
the obvious solution.

Instead of trying to filter Article.related_content.all(), modify the
default manager so that you can invoke
Articles.related_content.active().

To do this, define a custom default manager for Content that exposes
an 'active()' modifier. Rather than affecting all queries (by
overriding get_query_set()), you add an 'active()' operator that
returns a filtered query set. With this approach, both
Articles.related_content.active() and ContentGeneric.objects.active()
will both work.

Yours
Russ Magee %-)

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