docs

apps/api/src/exchanges/dto/exchange.dto.ts

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+import { Type } from 'class-transformer';
+import {
+  ArrayMaxSize,
+  ArrayMinSize,
+  IsArray,
+  IsInt,
+  IsString,
+  Min,
+  ValidateNested,
+} from 'class-validator';
+
+export class ExchangeItemDto {
+  @IsString()
+  itemKey!: string;
+
+  @IsInt()
+  @Min(1)
+  quantity!: number;
+}
+
+export class ExchangeRequestDto {
+  /**
+   * Item keys and quantities only.
+   *
+   * Prices and rewards are never accepted from the client -- the server reads
+   * them from the exchange rules (slice §8, NPC spec §37.9). The upper bound
+   * keeps one request from turning into an unbounded row-by-row transaction.
+   */
+  @IsArray()
+  @ArrayMinSize(1)
+  @ArrayMaxSize(50)
+  @ValidateNested({ each: true })
+  @Type(() => ExchangeItemDto)
+  items!: ExchangeItemDto[];
+}