Mongo Embedded Document is not JSON Serializable - Python - json

Question

I am currently using Graphene with MongoEngine. The mongo db schemas are as follows
class DocumentAModel(Document):
id = StringField(required=True)
documentB = MapField(EmbeddedDocumentField(DocumentBModel)
class DocumentBModel(EmbeddedDocument):
id = StringField(required=True)
value = IntField()
A sample documentA would be as following
{
id: "id_1",
documentB: {
0: {
id: "b_1",
value: 1
},
1: {
id: "b_2",
value: 11
}
}
And correspondingly, their Graphene types are
class DocumentB(MongoengineObjectType):
class Meta:
model = DocumentBModel
class DocumentA(MongoengineObjectType):
class Meta:
model = DocumentAModel
Finally, the query looks like the following
class Query(graphene.ObjectType):
all_document_a = graphene.List(DocumentA)
def resolve_all_document_a(self, info):
return list(DocumentAModel.objects.all())
However, when I query allDocumentA to get document B, i get the error
Object of type DocumentBModel is not JSON serializable
I am not sure where to marshal Document B to json.
If I change DocumentB from MapField(EmbeddedDocumentField(DocumentBModel) to DictField(), it works without issue. But is there a way to use MapField?
Thanks

Answer 1

MapField is Similar to a DictField, except the ‘value’ of each item must match the specified field type, and keys must be String type.
So in your case DocumentAModel documentB field is MapField and type of value is DocumentBModel. So for DocumentBModel you need to provide id and value fields. You are creating this using graphene but the model mapping will be same for normal(REST) DRF API and GraphQL API. MapField has some validations like key should be String type and value should be type which you have mentioned in model, but in case of Dictfield, it does not needed such type validations, its just normal Python Dictionary field.
Check the code snippet. And change the Graphene query and schema as per requirement.
Check the following code:
class DocumentBModel(fields.EmbeddedDocument):
id = fields.StringField(required=True)
value = fields.IntField()
class DocumentAModel(Document):
name = fields.StringField(required=True)
documentB = fields.MapField(fields.EmbeddedDocumentField(DocumentBModel))
Django shell
$ python manage.py shell
>>>
>>> B_obj1 = DocumentBModel(**{'id': 'b_1', 'value': 1})
>>> B_obj2 = DocumentBModel(**{'id': 'b_2', 'value': 2})
>>> data_obj = DocumentAModel.objects.create(**{"name":"akash", "documentB":{"0":B_obj1, "1":B_obj2}})
>>> data_obj._data
{'id': ObjectId('5ebd1d2cf549becd5a462924'), 'name': 'akash', 'documentB': {'0': <DocumentBModel: DocumentBModel object>, '1': <DocumentBModel: DocumentBModel object
>}}
Database entry:
{
"_id" : ObjectId("5ebd1d2cf549becd5a462924"),
"name" : "akash",
"documentB" : {
"0" : {
"id" : "b_1",
"value" : 1
},
"1" : {
"id" : "b_2",
"value" : 2
}
}
}

Answer 2

I think the issue is because graphql requires the response object to have a definite structure. Having a dict or a map as an attribute "confuses" graphql.
In the example above, the query could be
query {
documentA {
id
documentB {
0 {
id
value
}
1 {
id
value
}
}
}
}
but documentB doesnt have any attributes 0 and 1. So it just have to be a list of dicts. In which case, the query will become
query {
documentA {
id
documentB {
id
value
}
}
}