Truncating a list in MongoDb

Question

I am using the c# driver, but I would be happy about pointers in any language.

My documents have the following structure:

 class Document
 {
      List<Comment> comments;
 }

Or in Json:

 [{
     "comments" : [{"comment" : "text1"}, {"comment" : "text2"}, ...]
  },
  {
     "comments" : [{"comment" : "text1"}, {"comment" : "text2"}, ...]
  }, ...]

As you can see, each document contains a list of comments.

My goal is to run a periodic task, that truncates the list of comments of each document to a specific number of elements (eg. 10).

The obvious way that comes to my mind is to:

• Fetch each document
• Get the comments that should be removed
• Update the document by it's id by pulling the ids of the comments that should be removed

Is there a possibility to do this with a bulk Update?

I couldn't think of a condition for the update that would me allow to truncate the number of comments without fetching them first.

Answer 1

You can slice the elements of the comments array to the last n elements (-10 in the example below). Try this in the shell:

db.coll.update(
  { },
  { $push: { comments: { $each: [ ], $slice: -10 } } },
  { multi: true }
)

Since MongoDB 2.6 you can also use a positive n to update the array to contain only the first n elements.

In case you have a field you want to sort on before applying the slice operation:

db.coll.update(
  { }, { 
    $push: { 
      comments: { 
        $each: [ ], 
        $sort: { <field_to_sort_on>: 1 },
        $slice: -10 
      } 
    } 
  },
  { multi: true }
)