Polymorphic Schemas - MongoDB Applied Design Patterns - page 21

Relational databases typically support migrations via the ALTER TABLE statement, which al-
lows the developer to add or remove columns from a table. For instance, suppose we wanted
to add a short description field to our nodes table from Table 2-1 . The SQL for this opera-
tion would be similar to the following:

ALTER ALTER TABLE TABLE nodes

ADD ADD COLUMN COLUMN short_description varchar ( 255 );

The main drawbacks to the ALTER TABLE statement is that it can be time consuming to run on
a table with a large number of rows, and may require that your application experience some
downtimewhilethemigration executes, sincethe ALTER TABLE statement needstoholdalock
that your application requires to execute.

In MongoDB, we have the option of doing something similar by updating all documents in a
collection to reflect a new field:

db . nodes . update (
{},
{ $set : { short_description : '' } },
false

false , // upsert
true true // multi

);

This approach, however, has the same drawbacks as an ALTER TABLE statement: it can be
slow, and it can impact the performance of your application negatively.

Another option for MongoDB users is to update your application to account for the absence
of the new field. In Python, we might write the following code to handle retrieving both “old
style” documents (without a short_description field) as well as “new style” documents
(with a short_description field):

def def get_node_by_url ( url ):

node = db . nodes . find_one ({ 'url' : url })
node . setdefault ( 'short_description' , '' )
return

return node