Mapping field.

[amcgregor]

The addition of a Mapping field (a specialization of Array) to allow retrieval and storage of an ordered mapping of objects utilizing a common key field.

Specifically, the following BSON document:

{things: [{name: 'bob', age: 22}, {name: 'alice', potato: true}]}

Would be retrievable:

class Thing(Document):
    class Person(Document):
        name = String()
        age = Integer()
        potato = Boolean()
    
    things = Mapping('.Person')

Thing.find_one().things['bob'].age == 22

Work has already begun on this feature:

• b7c64ec Added mapping field and update localized to utilize it.

[amcgregor]

Test coverage complete. As a very explicit note, this is not a special change-tracking mapping; it is recommended to ensure use is either singular, or that you acquire a handle in your local namespace to the attribute's value prior to operating upon it; the mapping is constructed on access so as to preserve the in-database representation within the underlying datastore.