I would take this a step further.
Embed your OrderDetails into your Orders. Also, I would convert the names to the Rails convention:
class OrderDetail
include MongoMapper::EmbeddedDocument
key :order_number, String
key :product_code, String
key :quantity_ordered, Integer
key :order_number, String
key :order_date, Time
key :require_date, Time
key :shipped_date, Time
many :order_details
end
This is a more typical way of organizing this sort of data in Mongo. You'll get better performance (only one query) and it's easy to query by order_details if needed. If you need to export data, it's all in one collection.
You'll find in a Mongo db that you have fewer collections than you would tables in a Relational SQL db. If not, you're likely either using Mongo poorly or should be using a Relational SQL db anyway.
In terms of migrating your data, you'll just need a script that pulls data from your Relational DB converts the data into Mongo data structures and dumps them in Mongo. I'd skip Rails and MongoMapper for this. Just a simple script in your favorite language that can stream or batch your data.
Pick your favorite Mongo driver (I'd guess Ruby since you're moving to MM) and use that.
Cheers,
Jamie