This endpoint, ('/district'), will query against the District model using session.query(District).all():

@app.route('/nba/api/v0.1/district', methods=['GET'])
def get_districts():
  districts = session.query(District).all()
  if 'geometry' in request.args.keys() and request.args['geometry'] in ('1','True'):
    data = [{"type": "Feature", 
    "properties":{"representative":district.name, "district":district.district,
 "state": district.state_ref.name, "id":district.id}, 
    "geometry":{"type":"MultiPolygon", 
    "coordinates":shapely.geometry.geo.mapping(to_shape(district.geom))["coordinates"]},
    } for district in districts]
  else:
    data = [{"type": "Feature", 
    "properties":{"representative":district.name, "district":district.district,
    "state": district.state_ref.name, "id":district.id}, 
    "geometry":{"type":"MultiPolygon", 
    "coordinates":["Truncated"]},
    } for district in districts]
  return jsonify({"type": "FeatureCollection","features":data})