TypeGraphQL中使用ethers.js BigNumber类型触发CannotDetermineGraphQLTypeError错误的技术问询

The error pops up because TypeGraphQL doesn't natively recognize ethers.js's BigNumber type. Unlike standard TypeScript types or built-in GraphQL scalars, custom types like BigNumber need a dedicated GraphQL scalar definition to work seamlessly with TypeGraphQL. Here are two solid solutions:

Option 1: Create a Custom GraphQL Scalar for BigNumber (Recommended) #

This approach lets TypeGraphQL fully understand BigNumber, handling serialization/deserialization automatically between your API and clients.

Step 1: Implement the Custom Scalar #

Make a scalar class to handle conversions between BigNumber and GraphQL's string representation (since GraphQL doesn't have a native big number type):

import { Scalar, CustomScalar } from 'type-graphql';
import { ValueNode, Kind } from 'graphql';
import { BigNumber } from 'ethers';

@Scalar('BigNumber')
export class BigNumberScalar implements CustomScalar<string, BigNumber> {
  description = 'Custom scalar type for ethers.js BigNumber values';

  // Convert BigNumber to string for GraphQL output
  serialize(value: BigNumber): string {
    if (!(value instanceof BigNumber)) {
      throw new Error('BigNumberScalar can only serialize BigNumber instances');
    }
    return value.toString();
  }

  // Convert incoming string value to BigNumber
  parseValue(value: string): BigNumber {
    return BigNumber.from(value);
  }

  // Parse GraphQL AST literal to BigNumber
  parseLiteral(ast: ValueNode): BigNumber {
    if (ast.kind === Kind.STRING) {
      return BigNumber.from(ast.value);
    }
    throw new Error('BigNumberScalar only supports string literals');
  }
}

Step 2: Use the Scalar in Your Entity #

Update your Transfer entity to reference the custom scalar in the @Field decorator:

@ObjectType()
@Entity()
export class Transfer extends BaseEntity {
  @Field(() => BigNumberScalar) // Explicitly specify the custom scalar
  @Column({ type: "numeric", precision: 78, scale: 0, transformer: new BigNumberTransformer() })
  amount!: BigNumber;
}

Step 3: Register the Scalar with TypeGraphQL #

When building your GraphQL schema, register the scalar so TypeGraphQL recognizes it:

import { buildSchema } from 'type-graphql';
import { BigNumberScalar } from './BigNumberScalar';
import { Transfer } from './Transfer';

async function bootstrap() {
  const schema = await buildSchema({
    resolvers: [/* Your resolvers here */],
    entities: [Transfer],
    scalarsMap: [{ type: BigNumber, scalar: BigNumberScalar }],
  });
  // Use the schema with your server (Apollo, etc.)
}

Option 2: Explicitly Map BigNumber to GraphQL String Type #

If you don't want to create a custom scalar, you can directly tell TypeGraphQL to treat amount as a string. This works because your TypeORM transformer already converts BigNumber to/from strings for the database:

@ObjectType()
@Entity()
export class Transfer extends BaseEntity {
  @Field(() => String) // Force GraphQL to output a string
  @Column({ type: "numeric", precision: 78, scale: 0, transformer: new BigNumberTransformer() })
  amount!: BigNumber;
}

Note: With this approach, your API clients will receive the value as a string, and they'll need to convert it back to a BigNumber using ethers.BigNumber.from() if needed.

Key Notes #

• Your existing BigNumberTransformer for TypeORM is still valid and handles database conversion — these solutions only fix the TypeGraphQL type mapping.
• If you're using NestJS with @nestjs/graphql, register the scalar as a provider in your module instead of using scalarsMap:

  @Module({
    providers: [BigNumberScalar],
  })
  export class YourModule {}
  

内容的提问来源于stack exchange，提问作者sev