MongoEngineConversionError: Don't know how to convert the MongoEngine field <class 'mongoengine.fields.StringField'> (<class 'type'>)

[Ahmedn1]

When I have a list field like this:
some_field = graphene.List(graphene.String)
Then I build a MongoEngineObjectType for it:

class new_ObjectType(MongoengineObjectType):
    class Meta:
        model = BaseObjectType

I get this error because this method convert_field_to_list calls this method convert_mongoengine_field which always raises the exception.

[miguescri]

I encountered a problem that resulted in the MongoEngineConversionError and when googling this was the only issue/post I found, so I'll add some clarifications for future reference.

The graphene_mongo.converter.convert_mongoengine_field function is generic; it is decorated with functools.singledispatch and all the other functions in graphene_mongo.converter are implementations of convert_mongo_engine_field for different mongoengine field types. The only way to raise the MongoEngineConversionError exception inside convert_mongo_engine_field is by providing an argument that is not recognized by any of the supported mongoengine fields.

The convert_field_to_list implementation recursively calls convert_mongoengine_field to convert the type of items that the list holds, which may be any kind of mongoengine fields (lists included). Again, this will only raise the exception if the type of the list's contents is not supported.

Regarding the original question, the Meta.model attribute in a MongoengineObjectType needs to inherit from mongoengine.Document. If you are providing graphene field types inside a mongoengine.Document or providing Meta.model with a graphene.ObjectType, it will break because won't be able to do the conversion.

[abawchen]

@miguescri : Thanks for your explanation -- it really helps 👍
@Ahmedn1 : Does it answer your question? i.e the Meta.model attribute in a MongoengineObjectType needs to inherit from mongoengine.Document

[leonardwellthy]

I managed to reproduce this problem with the exact same error message, but a different root cause. So for future Googlers, here's my example:

from graphene_mongo import MongoengineObjectType
from mongoengine import Document, ListField, StringField


class ExampleDocumentModel(Document):
    example_list_field = ListField(StringField)


class ExampleType(MongoengineObjectType):
    class Meta:
        model = ExampleDocumentModel

and the problem is that a Mongoengine ListField takes another field object as its parameter, not a field class. Which is exactly what the error message is saying, I just couldn't interpret it without lots of trial and error.

So the corrected Mongoengine model should look like:

class ExampleDocumentModel(Document):
    example_list_field = ListField(StringField())