Custom GraphQL resolvers

One can extend the GraphQL API generated by OpenReader with custom queries. To do that, one can define GraphQL query resolvers in the designated module src/server-extension/resolvers. Note that all resolver classes (including any additional types) must be exported by src/server-extension/resolvers/index.ts.

A custom resolver should import TypeGraphQL types and use annotations provided by the library to define query arguments and return types.

Custom resolvers are normally used in combination with TypeORM EntityManager for accessing the API server target database. It is automatically injected when defined as a single constructor argument of the resolver.

A custom resolver may then look as follows:

import { Arg, Field, ObjectType, Query, Resolver } from 'type-graphql'
import type { EntityManager } from 'typeorm'
import { MyEntity } from '../../model/generated'

// Define custom GraphQL ObjectType of the query result
@ObjectType()
export class MyQueryResult {
  @Field(() => Number, { nullable: false })
  total!: number

  @Field(() => Number, { nullable: false })
  max!: number

  constructor(props: Partial<MyQueryResult>) {
    Object.assign(this, props);
  }
}

@Resolver()
export class MyResolver {
  // Set by depenency injection
  constructor(private tx: () => Promise<EntityManager>) {}

  @Query(() => [MyQueryResult])
  async myQuery(): Promise<MyQueryResult[]> {
    const manager = await this.tx()
    // execute custom SQL query
    const result: = await manager.getRepository(MyEntity).query(
      `SELECT 
        COUNT(x) as total, 
        MAX(y) as max
      FROM my_entity 
      GROUP BY month`)
    return result
  }
}

Some great examples of @subsquid/graphql-server-based custom resolvers can be spotted in the wild in the Rubik repo by KodaDot.

For more examples of resolvers, see TypeGraphQL examples repo.