Working on a project where I have mongoose schema with a field typed with Record<string, string>; I need graphql to understand that particular type (since Object is too “vague”), but I keep encountering the following issue:

CannotDetermineOutputTypeError: Cannot determine a GraphQL output type for the “measurements”. Make sure your class is decorated with an appropriate decorator.

Here is the field definition if the @Schema annotated schema class:

  @Prop({ type: Map, of: String, default: {} })
  @Field(() => StringRecordScalar)
  measurements: Record<string, string>;

This is the definition of StringRecordScalar:

import { Scalar, CustomScalar } from '@nestjs/graphql';
import { Kind, ValueNode, ObjectValueNode } from 'graphql';

@Scalar('StringRecordScalar', () => Object)
export class StringRecordScalar
  implements CustomScalar<any, Record<string, string>>
{
  description = 'Custom scalar for Record<string, string>';

  // Serialize the data (output to the client)
  serialize(value: Record<string, string>): Record<string, string> {
    if (typeof value !== 'object' || Array.isArray(value)) {
      throw new Error('Invalid Record<string, string> output');
    }
    return value;
  }

  // Parse the input value from the client (input type)
  parseValue(value: any): Record<string, string> {
    if (typeof value !== 'object' || Array.isArray(value)) {
      throw new Error('Invalid Record<string, string> input');
    }
    // Ensure all keys/values are strings
    Object.keys(value).forEach((key) => {
      if (typeof value[key] !== 'string') {
        throw new Error('All values in Record<string, string> must be strings');
      }
    });
    return value;
  }

  // Parse literal values from the GraphQL query itself
  parseLiteral(ast: ValueNode): Record<string, string> {
    if (ast.kind !== Kind.OBJECT) {
      throw new Error('Invalid Record<string, string> literal');
    }

    const result: Record<string, string> = {};
    (ast as ObjectValueNode).fields.forEach((field) => {
      if (field.value.kind === Kind.STRING) {
        result[field.name.value] = field.value.value;
      } else {
        throw new Error(
          `Expected value of type String for key "${field.name.value}"`,
        );
      }
    });

    return result;
  }
}