Proper ValidationError when trying to save a single doc as a list of EmbeddedDocuments

[wojcikstefan]

In [8]: class EmbeddedDoc(EmbeddedDocument):
   ...:     name = StringField()
   ...:

In [9]: class Doc(Document):
   ...:     docs = ListField(EmbeddedDocumentField(EmbeddedDoc))
   ...:

In [10]: Doc.objects.create(docs=[EmbeddedDoc(name='whatever')])  # works fine
Out[10]: <Doc: Doc object>

In [11]: Doc.objects.create(docs=EmbeddedDoc(name='whatever'))  # uh oh, assigning a single doc to a list?!
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-11-226358097c9d> in <module>()
----> 1 Doc.objects.create(docs=EmbeddedDoc(name='whatever'))  # uh oh, assigning a single doc to a list?!

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/queryset/base.pyc in create(self, **kwargs)
    286         .. versionadded:: 0.4
    287         """
--> 288         return self._document(**kwargs).save()
    289
    290     def first(self):

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/base/document.pyc in __init__(self, *args, **values)
    113                         field = self._fields.get(key)
    114                         if field and not isinstance(field, FileField):
--> 115                             value = field.to_python(value)
    116                     setattr(self, key, value)
    117                 else:

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/base/fields.pyc in to_python(self, value)
    315             self.field._auto_dereference = self._auto_dereference
    316             value_dict = {key: self.field.to_python(item)
--> 317                           for key, item in value.items()}
    318         else:
    319             Document = _import_class('Document')

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/base/fields.pyc in <dictcomp>((key, item))
    315             self.field._auto_dereference = self._auto_dereference
    316             value_dict = {key: self.field.to_python(item)
--> 317                           for key, item in value.items()}
    318         else:
    319             Document = _import_class('Document')

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/fields.pyc in to_python(self, value)
    544     def to_python(self, value):
    545         if not isinstance(value, self.document_type):
--> 546             return self.document_type._from_son(value, _auto_dereference=self._auto_dereference)
    547         return value
    548

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/base/document.pyc in _from_son(cls, son, _auto_dereference, only_fields, created)
    679
    680         if son and not isinstance(son, dict):
--> 681             raise ValueError("The source SON object needs to be of type 'dict'")
    682
    683         # Get the class name from the document, falling back to the given

ValueError: The source SON object needs to be of type 'dict'

ValueError: The source SON object needs to be of type 'dict' isn't very helpful when debugging this subtle issue.

With simple types it works as expected, raising a ValidationError with a clear message:

In [2]: class Doc(Document):
   ...:     lst = ListField(StringField())
   ...:

In [5]: Doc.objects.create(lst='1')
(...)
ValidationError: ValidationError (Doc:None) (Only lists and tuples may be used in a list field: ['lst'])

Thanks to #1460 for exposing it.