TypeGraphql 官方文档示例练习：

import {
  ObjectType,
  Field,
  ID,
  Int,
  Resolver,
  Query,
  Args,
  ArgsType,
  InputType,
  Mutation,
  Arg,
  FieldResolver,
  Root
} from 'type-graphql'
import { Min, Max } from 'class-validator'

// 定义类型和字段
@ObjectType({ description: 'test' })
class Recipe {
  @Field(() => ID, { nullable: true })
  id?: string

  @Field()
  title: string

  @Field({ nullable: true })
  description?: string

  @Field()
  ratings?: number[]

  @Field()
  averageRating?: number
}

// 定义 Mutation 用到的参数类型
@InputType()
class AddRecipeInput implements Partial<Recipe> {
  @Field()
  title: string

  @Field({ nullable: true })
  description?: string
}

// 定义一个 Query 用到的参数类型
@ArgsType()
class GetRecipesArgs {
  // 使用defaultValue，TypeGraphql会设置默认值并使该字段可为空
  @Field(() => Int, { defaultValue: 0 })
  @Min(0)
  skip: number

  @Field(() => Int)
  @Min(1)
  @Max(50)
  take = 25 // 属性初始化，和 defaultValue 的效果一样

  @Field({ nullable: true })
  title?: string

  // 可以在 args 中自定义辅助方法
  get startIndex(): number {
    return this.skip
  }

  get endIndex(): number {
    return this.skip + this.take
  }
}

// 创建一个解析器
@Resolver()
class RecipeResolver {
  // 使用依赖注入，将服务器/存储库/数据存储在resolver类的内部，保证每个应用都由唯一的实例
  private recipesCollection: Recipe[] = []

  // 创建一个 Query
  @Query(() => [Recipe])
  async recipes(
    @Args()
    { title, startIndex, endIndex }: GetRecipesArgs
  ): Promise<Recipe[]> {
    let recipes = this.recipesCollection
    if (title) {
      recipes = recipes.filter((recipe) => recipe.title === title)
    }
    return recipes.slice(startIndex, endIndex)
  }

  // 创建一个 Mutation
  @Mutation()
  addRecipe(
    @Arg('data')
    newRecipeData: AddRecipeInput
    // @Ctx()
    // ctx: Context
  ): Recipe[] {
    this.recipesCollection.push(newRecipeData)
    return this.recipesCollection
  }

  // 创建一个字段解析器
  @FieldResolver()
  averageRating(
    @Root()
    recipe: Recipe
  ) {
    const ratingsSum =
      recipe?.ratings?.reduce((prev, next) => prev + next, 0) || 0
    return recipe?.ratings?.length ? ratingsSum / recipe.ratings.length : null
  }
}