Problem details

Every Django model has a default manager called objects. Invoking objects.all(), will return all the entries for that model in the database. Usually, we are interested in only a subset of all entries.

We apply various filters to find out the set of entries we need. The criterion to select them is often our core business logic. For example, we can find the posts accessible to the public by the following code:

public = Posts.objects.filter(privacy="public") 

This criterion might change in the future. For example, we might want to also check whether the post was marked for editing. This change might look as follows:

public = Posts.objects.filter(privacy=POST_PRIVACY.Public, 
         draft=False) 

However, this change needs to be made everywhere a public post is needed. This can get very frustrating. There needs to be only one place to define such commonly used queries without repeating oneself.