This allows users of Mongoid to define custom field options, and execute
handlers when that option is provided.

A few use-cases spring to mind:

Auto-validations

It should be (fairly) trivial to implement field-level validations like
DataMapper and ActiveRecord, with the most trivial example looking
something like this.

Mongoid::Field.option :required do |model, field, required|
model.validates_presence_of field.name if required
end

Now the block will be executed whenever a field is defined that includes
the key :required in its options:

Person.field :name, required: true
Person.field :age, required: false

All options provided are still stored on the Field, which should also
allow for a richer level of introspection:

class Person
def required_fields
fields.select

{ |field| field.options[:required] }

end
end

Mass Assignment

Mongoid::Field.option :accessible do |model, field, accessible|
model.attr_accessible field.name if accessible
end

Re-writing option values, and other uses

1. Turn :alias => :city into :alias => [:city]
   Mongoid::Field.option :alias do |model, field|
   field.options[:aliases] = [*field.options.delete(:alias)]
   end

Mongoid::Field.option :aliases do |model, field, aliases|
aliases.each do |new_name|
model.alias_attribute new_name, field.name
end
end