can't use type 'object' for Type-GraphQL field types - graphql

Question

I would like to create a GraphQL layer for my NestJs API. So based on this interface holding a key/value pairs
export interface Mapping {
id: string;
value: object;
}
I created a DTO acting as a return type for the GraphQL queries
#ObjectType()
export class MappingDTO {
#Field(() => ID)
id: string;
#Field()
value: object;
}
So when I want to find all mappings I would come up with this
#Query(() => [MappingDTO])
public async mappings(): Promise<Mapping[]> {
return this.mappingsService.getMappings();
}
Unfortunately I can't use object for the value field. I'm getting this error
NoExplicitTypeError: You need to provide explicit type for
MappingDTO#value !
When chaning the type e.g. to string no error gets thrown on startup. The thing is that I can't specify an explicit type because my MongoDB stores objects holding "anything".
How can I fix my resolver or DTO to deal with an object type?
Edit:
Same for the resolver decorator. When I have this
#Query(() => Object)
public async getValueByKey(#Args('id') id: string): Promise<object> {
return this.mappingsService.getValueByKey(id);
}
I get this error
Error: Cannot determine GraphQL output type for getValueByKey

Answer 1

Sounds like you should be using a custom JSON scalar. Like this one.
import { GraphQLJSONObject } from 'graphql-type-json'
This can then either be used directly in the field decorator:
#Field(() => GraphQLJSONObject)
value: object
or you can leave the decorator as #Field() and add the appropriate scalarsMap to the options you pass to buildSchema. See the docs for additional details.

Answer 2

Just to point out the fact that the whole idea of GraphQL is declarative data fetching.
Also you can define custom types in GraphQL the scalar types don't have object as option, but you can define a type of yourself like this.
type customObject {
keyOne: Int!
KeyTwo: String!
...
key(n): ...
}
and use this type in Interfaces, Query, Mutations:
export interface Mapping {
id: string;
value: customObject;
}
You might think that you will have to define every key of the object you are storing in the mongoDB in this type customObject, but you don't have to. this is where the GraphQL declarative data fetching comes in. You only define those fields in the type (customObject) that you want to use on the front end, you don't need to fetch the complete object from DB.