Field order not preserved during redefinition.

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.

No source file or test is named in the issue. Start by tracing how inherited fields are collected into Page._fields_ordered, using the supplied Asset/Page example; done means the redefined handler retains its original position while the other fields remain ordered.