feat(logs): Log duplicate handlers

src/command-bus.ts

@@ -1,4 +1,4 @@
-import { Injectable, Type } from '@nestjs/common';
+import { Injectable, Logger, Type } from '@nestjs/common';
 import { ModuleRef } from '@nestjs/core';
 import 'reflect-metadata';
 import {
@@ -26,6 +26,7 @@ export class CommandBus<CommandBase extends ICommand = ICommand>
 {
   private handlers = new Map<string, ICommandHandler<CommandBase>>();
   private _publisher: ICommandPublisher<CommandBase>;
+  private readonly _logger = new Logger(CommandBus.name);
 
   constructor(private readonly moduleRef: ModuleRef) {
     super();
@@ -64,7 +65,17 @@ export class CommandBus<CommandBase extends ICommand = ICommand>
     return handler.execute(command);
   }
 
-  bind<T extends CommandBase>(handler: ICommandHandler<T>, id: string) {
+  bind<T extends CommandBase>(
+    handler: ICommandHandler<T>,
+    { id, name }: CommandMetadata,
+  ) {
+    if (this.handlers.has(id)) {
+      const previousHandlerName = this.handlers.get(id).constructor.name;
+      const handlerName = handler.constructor.name;
+      this._logger.warn(
+        `Multiple handlers for command ${name}. Repleacing ${previousHandlerName} with ${handlerName}.`,
+      );
+    }
     this.handlers.set(id, handler);
   }
 
@@ -77,11 +88,11 @@ export class CommandBus<CommandBase extends ICommand = ICommand>
     if (!instance) {
       return;
     }
-    const target = this.reflectCommandId(handler);
-    if (!target) {
+    const commandMetadata = this.reflectCommandMetadata(handler);
+    if (!commandMetadata) {
       throw new InvalidCommandHandlerException();
     }
-    this.bind(instance as ICommandHandler<CommandBase>, target);
+    this.bind(instance as ICommandHandler<CommandBase>, commandMetadata);
   }
 
   private getCommandId(command: CommandBase): string {
@@ -97,16 +108,12 @@ export class CommandBus<CommandBase extends ICommand = ICommand>
     return commandMetadata.id;
   }
 
-  private reflectCommandId(handler: CommandHandlerType): string | undefined {
+  private reflectCommandMetadata(handler: CommandHandlerType): CommandMetadata {
     const command: Type<ICommand> = Reflect.getMetadata(
       COMMAND_HANDLER_METADATA,
       handler,
     );
-    const commandMetadata: CommandMetadata = Reflect.getMetadata(
-      COMMAND_METADATA,
-      command,
-    );
-    return commandMetadata.id;
+    return Reflect.getMetadata(COMMAND_METADATA, command);
   }
 
   private useDefaultPublisher() {

src/decorators/command-handler.decorator.ts

@@ -2,6 +2,7 @@ import 'reflect-metadata';
 import { ICommand } from '../index';
 import { COMMAND_HANDLER_METADATA, COMMAND_METADATA } from './constants';
 import { v4 } from 'uuid';
+import { Type } from '@nestjs/common';
 
 /**
  * Decorator that marks a class as a Nest command handler. A command handler
@@ -13,10 +14,14 @@ import { v4 } from 'uuid';
  *
  * @see https://docs.nestjs.com/recipes/cqrs#commands
  */
-export const CommandHandler = (command: ICommand | (new (...args: any[]) => ICommand)): ClassDecorator => {
+export const CommandHandler = (command: Type<ICommand>): ClassDecorator => {
   return (target: object) => {
     if (!Reflect.hasOwnMetadata(COMMAND_METADATA, command)) {
-      Reflect.defineMetadata(COMMAND_METADATA, { id: v4() }, command);
+      Reflect.defineMetadata(
+        COMMAND_METADATA,
+        { id: v4(), name: command.name },
+        command,
+      );
     }
     Reflect.defineMetadata(COMMAND_HANDLER_METADATA, command, target);
   };

src/decorators/query-handler.decorator.ts

@@ -2,6 +2,7 @@ import 'reflect-metadata';
 import { IQuery } from '../interfaces';
 import { QUERY_HANDLER_METADATA, QUERY_METADATA } from './constants';
 import { v4 } from 'uuid';
+import { Type } from '@nestjs/common';
 
 /**
  * Decorator that marks a class as a Nest query handler. A query handler
@@ -13,10 +14,14 @@ import { v4 } from 'uuid';
  *
  * @see https://docs.nestjs.com/recipes/cqrs#queries
  */
-export const QueryHandler = (query: IQuery): ClassDecorator => {
+export const QueryHandler = (query: Type<IQuery>): ClassDecorator => {
   return (target: object) => {
     if (!Reflect.hasOwnMetadata(QUERY_METADATA, query)) {
-      Reflect.defineMetadata(QUERY_METADATA, { id: v4() }, query);
+      Reflect.defineMetadata(
+        QUERY_METADATA,
+        { id: v4(), name: query.name },
+        query,
+      );
     }
     Reflect.defineMetadata(QUERY_HANDLER_METADATA, query, target);
   };

src/interfaces/commands/command-metadata.interface.ts

@@ -1,3 +1,4 @@
 export interface CommandMetadata {
   id: string;
-}
+  name: string;
+}

src/interfaces/queries/query-metadata.interface.ts

@@ -1,3 +1,4 @@
 export interface QueryMetadata {
   id: string;
-}
+  name: string;
+}

src/query-bus.ts

@@ -1,4 +1,4 @@
-import { Injectable, Type } from '@nestjs/common';
+import { Injectable, Logger, Type } from '@nestjs/common';
 import { ModuleRef } from '@nestjs/core';
 import 'reflect-metadata';
 import { QUERY_HANDLER_METADATA, QUERY_METADATA } from './decorators/constants';
@@ -27,6 +27,7 @@ export class QueryBus<QueryBase extends IQuery = IQuery>
 {
   private handlers = new Map<string, IQueryHandler<QueryBase, IQueryResult>>();
   private _publisher: IQueryPublisher<QueryBase>;
+  private readonly _logger = new Logger(QueryBus.name);
 
   constructor(private readonly moduleRef: ModuleRef) {
     super();
@@ -69,9 +70,16 @@ export class QueryBus<QueryBase extends IQuery = IQuery>
 
   bind<T extends QueryBase, TResult = any>(
     handler: IQueryHandler<T, TResult>,
-    queryId: string,
+    { id, name }: QueryMetadata,
   ) {
-    this.handlers.set(queryId, handler);
+    if (this.handlers.has(id)) {
+      const previousHandlerName = this.handlers.get(id).constructor.name;
+      const handlerName = handler.constructor.name;
+      this._logger.warn(
+        `Multiple handlers for query ${name}. Repleacing ${previousHandlerName} with ${handlerName}.`,
+      );
+    }
+    this.handlers.set(id, handler);
   }
 
   register(handlers: QueryHandlerType<QueryBase>[] = []) {
@@ -83,11 +91,14 @@ export class QueryBus<QueryBase extends IQuery = IQuery>
     if (!instance) {
       return;
     }
-    const target = this.reflectQueryId(handler);
-    if (!target) {
+    const queryMetadata = this.reflectQueryMetadata(handler);
+    if (!queryMetadata) {
       throw new InvalidQueryHandlerException();
     }
-    this.bind(instance as IQueryHandler<QueryBase, IQueryResult>, target);
+    this.bind(
+      instance as IQueryHandler<QueryBase, IQueryResult>,
+      queryMetadata,
+    );
   }
 
   private getQueryId(query: QueryBase): string {
@@ -103,18 +114,14 @@ export class QueryBus<QueryBase extends IQuery = IQuery>
     return queryMetadata.id;
   }
 
-  private reflectQueryId(
+  private reflectQueryMetadata(
     handler: QueryHandlerType<QueryBase>,
-  ): string | undefined {
+  ): QueryMetadata {
     const query: Type<QueryBase> = Reflect.getMetadata(
       QUERY_HANDLER_METADATA,
       handler,
     );
-    const queryMetadata: QueryMetadata = Reflect.getMetadata(
-      QUERY_METADATA,
-      query,
-    );
-    return queryMetadata.id;
+    return Reflect.getMetadata(QUERY_METADATA, query);
   }
 
   private useDefaultPublisher() {