Safe

In MongoDB, database writes/updates follow the “fire and forget” pattern. This means that the database will not wait for a response when you perform an update operation on a document. MongoMapper follows this same convention by default.

The down side of this strategy is if something goes wrong–such as a unique index constraint failing or an internal MongoDB error–an error won’t be raised. You may never know that your data isn’t being saved, until you try to read it and it’s not there.

Instead of just using the default “fire and forget” behavior on our save operation, passing the :safe option to save will force the driver to make sure the save succeeds and raise an error if it doesn’t.

user = User.new("duplicate@example.com")
user.save(:safe => true) # => Mongo::OperationFailure: 11000: E11000 duplicate key error index: mealsnap_dev.foos.$email_1  dup key: { : "duplicate@example.com@" }

You could also declare safe in your model in order to force all operations to be safe.

class User
  include MongoMapper::Document
  safe

  key :email, String
end

The safe option can be either a boolean or a Hash of options. For example, if you want to ensure that a highly critical write is committed to the journal on the majority of replica set members before save returns, you would use:

invoice = Invoice.new(:status => :paid)
invoice.save(:safe => {:j => true, :w => 'majority'})

You can also set these options when you declare your model:

class Invoice
  include MongoMapper::Document
  safe(:j => true, :w => 'majority')
end

For more information about the available options, see the documentation for the getLastError command.