Django model with embedded documents issues error: coercing to Unicode: need string or buffer, xxx found

Question

I am trying to define models in Django with a list of embedded documents using mongoengine to connect to my mongodb data. Here is the relevant part of my models.py:

class Feature(EmbeddedDocument):
   title = StringField(max_length=100, required=True)
   is_property = BooleanField(default=True)
   property_type = StringField(max_length=50, choices=PROPERTY_TYPE)

   def __unicode__(self):
      return self.title or u''

class Property(EmbeddedDocument):
   feature = EmbeddedDocumentField(Feature)
   value = StringField(max_length=2000, required=True)

   def __unicode__(self):
      return self.feature

class Concept(Document):
   title = StringField(max_length=200, required=True)
   properties = ListField(EmbeddedDocumentField(Property))

In console, when I try to create a new Concept, i do the following:

f = Feature(title='start time',property_type='Time')
p = Property(feature=f, value='6:30pm')
c = Concept(title='test', properties = [p])
c.save() #or c.properties[0]

I then get the following error:

c.properties
Out[36]: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/IPython/core/interactiveshell.py", line 2731, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-36-47a09d2ccd39>", line 1, in <module>
    c.properties
  File "/usr/lib/python2.7/dist-packages/IPython/core/displayhook.py", line 238, in __call__
    format_dict = self.compute_format_data(result)
  File "/usr/lib/python2.7/dist-packages/IPython/core/displayhook.py", line 150, in compute_format_data
    return self.shell.display_formatter.format(result)
  File "/usr/lib/python2.7/dist-packages/IPython/core/formatters.py", line 126, in format
    data = formatter(obj)
  File "/usr/lib/python2.7/dist-packages/IPython/core/formatters.py", line 447, in __call__
    printer.pretty(obj)
  File "/usr/lib/python2.7/dist-packages/IPython/lib/pretty.py", line 345, in pretty
    return self.type_pprinters[cls](obj, self, cycle)
  File "/usr/lib/python2.7/dist-packages/IPython/lib/pretty.py", line 529, in inner
    p.pretty(x)
  File "/usr/lib/python2.7/dist-packages/IPython/lib/pretty.py", line 360, in pretty
    return _default_pprint(obj, self, cycle)
  File "/usr/lib/python2.7/dist-packages/IPython/lib/pretty.py", line 480, in _default_pprint
    p.text(repr(obj))
  File "/usr/local/lib/python2.7/dist-packages/mongoengine-0.8.6-py2.7.egg/mongoengine/base/document.py", line 201, in __repr__
    u = self.__str__()
  File "/usr/local/lib/python2.7/dist-packages/mongoengine-0.8.6-py2.7.egg/mongoengine/base/document.py", line 212, in __str__
    return unicode(self).encode('utf-8')
TypeError: coercing to Unicode: need string or buffer, Feature found

Answer 1

I think you need to explicitly convert the Feature to unicode, because mongoengine calls .encode on the __unicode__ of your object:

return unicode(self).encode('utf-8')

Where self is an instance of Property, and unicode(self) would return your Feature instance. So something like this should work

class Property(...):
    # ...
    def __unicode__(self):
        return unicode(self.feature)