I had a little fight with Marshmallow and the ordered=True in class Meta. When ordered=True is set the fields should be outputed in the order specified in the property fields of the class Meta.

Example:

class AuthorSchema(ma.Schema):
    id = base_fields.Int(dump_only=True)

    absolute_url = ma.AbsoluteURLFor('api.author', id='<id>')

    links = ma.Hyperlinks({
        'self': ma.URLFor('api.author', id='<id>'),
        'collection': ma.URLFor('api.authors')
    })

    @post_dump(pass_many=True)
    def wrap(self, data, many):
        key = 'authors' if many else 'author'
        return {
            key: data
        }

    class Meta:
        fields = (
            'id',
            'name',
            'links',
            'absolute_url',
        )
        ordered = True

This was the output.

{
  "author": {
    "absolute_url": "http://localhost:8080/api/v4/authors/123",
    "id": 123,
    "links": {
      "collection": "/api/v4/authors/",
      "self": "/api/v4/authors/123"
    },
    "name": "Fred Douglass"
  }
}

After some Googeling I found out that the culprit was Flask.jsonify. It orders output alphabetically by default. The following setting switches this off.

app.config['JSON_SORT_KEYS'] = False

After this the output was in the specified order.

{
  "author": {
    "id": 123,
    "name": "Fred Douglass",
    "links": {
      "self": "/api/v4/authors/123",
      "collection": "/api/v4/authors/"
    },
    "absolute_url": "http://localhost:8080/api/v4/authors/123"
  }
}

Hope this saves someone else time.