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

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.