GeoAlchemy pg_function.geojson cannot be decoded with json.loads

Question

Given this function:

features being a list of SQLAlchemy objects

template just being a dictionary that represents a template for display content to the user

def get_feature_dictionary(features, template):

  feature_list = []

  print features

  for feature in features:

    for field in template.fields:
      if field.data_type == 'geometry':
          geometry = getattr(feature, field.name)
          geometry_string = db.session.scalar(pg_functions.geojson(getattr(feature, field.name)))

    this_feature = {
      "type": "Feature",
      "geometry": json.loads(geometry_string),
      "properties": {
        "id": feature.id,
        "created": feature.created,
        "status": feature.status
      }
    }

    feature_list.append(this_feature)

  return feature_list

I would like to return a GeoJSON FeatureCollection that looks like this:

{
  "type": "FeatureCollection",
  "features": [
    {
      "geometry": {
        "type":"GeometryCollection",
        "geometries":[
          {"type":"Point","coordinates":[2,0]}
        ]
      },
      "properties": {
        "created": "Tue, 03 Aug 2010 17:11:13 GMT", 
        "id": 1, 
        "status": "public"
      }, 
      "type": "Feature"
    }
  ], 
  "properties": {
    "features_per_page": "", 
    "status": {
      "code": 200, 
      "type": "OK"
    }, 
    "total_features": 1021, 
    "total_pages": ""
  }
}

And I can get this to print out:

{
  "type": "FeatureCollection",
  "features": [
    {
      "geometry": '{\"type\":\"GeometryCollection\",\"geometries\":[{\"type\":\"Point\",\"coordinates\":[2,0]},{\"type\":\"Polygon\",\"coordinates\":[[[0,0],[1,0],[1,1],[0,1],[0,0]]]}]}'
    ], 
  "properties": {
    "features_per_page": "", 
    "status": {
      "code": 200, 
      "type": "OK"
    }, 
    "total_features": 1021, 
    "total_pages": ""
  }
}

But what I'm ending up with is an error that reads:

TypeError: expected string or buffer

TypeError
TypeError: expected string or buffer

Traceback (most recent call last)
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
return self.wsgi_app(environ, start_response)
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
response = self.make_response(self.handle_exception(e))
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "[--removed--]/modules/api/views.py", line 116, in index
search = get_search_results(feature_type, conditions, FeatureType)
File "[--removed--]/modules/api/search.py", line 499, in get_search_results
data = get_feature_dictionary(search['results'], template)
File "[--removed--]/modules/api/utilities.py", line 241, in get_feature_dictionary
Display the sourcecode for this frameOpen an interactive python shell in this frame"geometry": json.loads(geometry_string),
File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 338, in loads
return _default_decoder.decode(s)
File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 365, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
TypeError: expected string or buffer

I've tried changing the type using str() but that just results in a:

ValueError: No JSON object could be decoded

I've tried this with both GeoAlchemy (pg_functions.geojson) and GeoAlchemy2 (ST_AsGeoJSON) with identical results. I know I'm close, but for some reason I'm not passing the scalar properly or something.

Any one have an idea of what I'm doing wrong?

Answer 1

I found that the reason I was coming up with the ValueError: No JSON object could be decoded error is because I wasn't checking to see if a value existed.

I changed this:

geometry_string = db.session.scalar(pg_functions.geojson(getattr(feature, field.name)))

to this:

geometry_string = db.session.scalar(pg_functions.geojson(getattr(feature, field.name))) or '{}'

and everything worked perfectly, json.loads() was tripping over an empty string and can't parse an empty string as JSON, it at minimum needs to an empty JSON object {}