2009-12-09

"Soft Delete" (aka "Logical Delete") in ORM

People often ask which ORM supports "Soft Delete" or "Logical Delete". Let's try to figure out what is it and whether ORM should internally support it.

"Soft Delete" is a way of removing business entities, which implies that we mark an entity as deleted instead of physical removing it from database. This approach is often used in Line of Business applications because of its several advantages:
  • It allows us to keep history for different  auditing sceneries. For example, somebody removed one document in the past from our workflow system, we surely want to be able to audit removing log and data removed document contained.
  • It allows to implement Recycle Bin approach in easy way. We'd like to be able to recycle any document removed in the past.
To implement "Soft Delete" feature in a simple case we should:
  • Create IsDeleted persistent property of bool type in all softly removable types.
  • Automatically filter all queries, i.e. automatically add Where(entity => !entity.IsDeleted) to each LINQ query.
In my example on DataObjects.Net I use single base class for all business objects, so I can add IsDeleted field to this class:

public class BusinessObject : Entity
{
  [Field]
  public bool IsDeleted { get; set;}

  public new void Remove()
  {
    IsDeleted = true;
  }
}
Then let's create DataContext class responsible for data access:

public static class DataContext
{
  public static IQueryable<T> GetAll<T>()
    where T : BusinessObject
  {
    return Query<T>.All
      .Where(entity => !entity.IsDeleted);
  }
}

Now we can softly remove our entities and query not removed ones. I've created small sample illustrating model of blog-publishing service, consisting of three classes: Blog, BlogPost and Comment. So I can query recent posts from my blog using such LINQ-query:

from post in DataContext.GetAll<BlogPost>()
where
  post.PublishDate > DateTime.Now-TimeSpan.FromDays(7) &&
  post.Blog == myBlog
select post;

This query will return all posts except softly deleted and I don't have to add appropriate check to every query in my application.

Generally, I am sure that "Soft Delete" shouldn't be internally implemented within an ORM framework, because it's easy to implement it yourself and your own implementation will be more flexible than built-in. Why flexibility is important? In my example I've chosen the simplest way of its implementation. In real-life scenarios there are many aspects connected with soft deletion in specific ways, for example:
  • Security: You may want to define who has a permission to access removed entities, etc...
  • Entities dependency: You may want to consider some entities as deleted when their dependency parent is deleted, so you need to specify more complex filter on queries.

2 comments:

Alex Yakunin said...

Btw, the APIs we've introduced for localization already allow to implement automatic filtering of soft-deleted items (i.e. you shouldn't build your special queryable for this). We must show this in samples later.

Anonymous said...

What a great resource!