Implement custom feature properties source

[lrvdijk]

By default, when converting a django model instance with geometry
to GeoJSON, the "properties" field will be filled with the fields
defined in the corresponding serializer.

In this PR we separate the code a bit in individual methods,
which can be overridden by any subclass. In this way the user is
able to define their own source for the properties field of the
GeoJSON output.

Solves #70

[lrvdijk]

Sample usage:

class NetworkGeoSerializer(GeoFeatureModelSerializer):
    class Meta:
        model = models.Link
        geo_field = 'geo'
        auto_bbox = True

    def get_feature_properties(self, instance):
        return instance.metadata  # This is a PostgreSQL HStore field, which django maps to a dict

    def unformat_geojson(self, feature):
        attribs = {
            self.Meta.geo_field: feature["geometry"],
            "metadata": feature["properties"]
        }

        if self.Meta.bbox_geo_field and "bbox" in feature:
            attribs[self.Meta.bbox_geo_field] = Polygon.from_bbox(
                feature["bbox"])

        return attribs

[pglotov]

Feels like this functionality belongs to ModelSerializer from DRF. GeoFeatureModelSerializer should just repackage fields, whatever they are that came after calling ModelSerializer. It is reasonable to have a situation like this arise with no regard to geolocation: serialize an object from a HStore as in the referred issue.

[lrvdijk]

Hmm true, but this is still a little bit different, because we still have the geometry (and optionally the bounding box) which is part of the model/serializer.

[landscape-bot]

Code quality remained the same when pulling f2f4c01 on sh4wn:custom-feature-properties into b5947a9 on djangonauts:master.

[nemesifier]

👍

I did not like the nested function declarations, a few months ago I also thought that they could make customization of the serializer more cumbersome.

For the moment I'll only be able to ask you 2 things:

1. a few style changes for which i'll leave inline comments
2. a few lines of documentation in the GeoFeatureModelSerializer section, in a new subsection right after Bounding Box: "auto_bbox" and "bbox_geo_field")

In a couple of weeks I'll test the feature properly and merge it if it's ready.

[nemesifier]

generally I do not like to enforce strict line breaking because in instances like this one I have a hard time reading the code. Could you please put Noneon the line above?

[landscape-bot]

Code quality remained the same when pulling f2f4c01 on sh4wn:custom-feature-properties into f58d9c5 on djangonauts:master.

[lrvdijk]

Sorry for the delay, but those things you've mentioned should be fixed now! :)

[nemesifier]

Thank you @sh4wn, I merged this manually in 5a82a00.

I needed to change a few bits in order to avoid losing some performance gains I managed to introduce in the latest versions, but the behaviour you introduced should be unchanged (except for the name of the method get_feature_properties which I renamed to get_properties for simplicity).

Please test 0.9.5 alpha in your app and report any feedback.