Field order not preserved during redefinition.

[amcgregor]

There's a use case where subclassing is used for specialization and overriding of default values.

Take this example:

from mongoengine import Document, StringField

class Asset(Document):
    meta = dict(allow_inheritance=True)
    handler = StringField(default="web.component.asset:AssetController")
    path = StringField()

class Page(Asset):
    handler = StringField(default="web.component.page:PageController")

However, there is a bit of an issue that crops up when you have defined multiple fields, then override one this way.

print(Page._fields_ordered)
# ('id', 'path', 'handler', '_cls')

My own declarative schema system uses this chunk of metaclass to avoid this issue: https://github.com/marrow/schema/blob/develop/marrow/schema/meta.py#L60-L95 (apologies for the metaclass bit being exceptionally difficult to grok Python code… nature of the beast.)

Very specifically, this explicitly preserves declaration order for redefined fields:

from marrow.schema import Container, Attribute

class Foo(Container):
    handler = Attribute()
    path = Attribute()

class Bar(Foo):
    handler = Attribute()

print(Bar.__attributes__)
# handler, path - order preserved despite the redefinition getting a later sequence ID

This metaclass example may provide an approach to resolve this issue.