DiscoverableMetaHostCollection

Kind: Class

Source: packages/core/discovery/discoverable-meta-host-collection.ts

Part of: Core

DiscoverableMetaHostCollection tracks Nest providers and controllers by discoverable metadata keys. It records class-to-metadata links, inspects framework-managed InstanceWrapper objects during discovery, and exposes indexed sets for metadata-based lookup.

Methods

Properties

Where it refuses work

• DiscoverableMetaHostCollection stops the work with an early return when !metaKey.

Diagram

mermaid
graph LR
  Decorator[Custom decorator] -->|addClassMetaHostLink| Links[Class metadata links]
  Provider[Provider InstanceWrapper] -->|inspectProvider| Collection[DiscoverableMetaHostCollection]
  Controller[Controller InstanceWrapper] -->|inspectController| Collection
  Links --> Collection
  Collection --> ProviderIndex[providersByMetaKey]
  Collection --> ControllerIndex[controllersByMetaKey]
  ProviderIndex -->|getProvidersByMetaKey| Result[Set of InstanceWrapper]
  ControllerIndex -->|getControllersByMetaKey| Result

Usage

ts
import { SetMetadata } from '@nestjs/common';
import { DiscoverableMetaHostCollection } from '@nestjs/core/discovery/discoverable-meta-host-collection';
import type { InstanceWrapper } from '@nestjs/core/injector/instance-wrapper';

const FEATURE_KEY = Symbol('feature');

// A custom discoverable decorator should register both metadata and the class link.
export function FeatureHandler(): ClassDecorator {
  return (target) => {
    SetMetadata(FEATURE_KEY, true)(target);

    DiscoverableMetaHostCollection.addClassMetaHostLink(
      target,
      FEATURE_KEY,
    );
  };
}

@FeatureHandler()
class ReportService {}

const collection = new DiscoverableMetaHostCollection();

// `providerWrapper` is normally supplied by Nest's module/container discovery flow.
const providerWrapper = {} as InstanceWrapper;
collection.inspectProvider(providerWrapper);

const handlers = collection.getProvidersByMetaKey(FEATURE_KEY);

for (const wrapper of handlers) {
  console.log(wrapper.metatype?.name);
}

AI Coding Instructions

• Register class metadata through addClassMetaHostLink() when creating discoverable class decorators; setting reflection metadata alone is not sufficient for this index.
• Use inspectProvider() for provider wrappers and inspectController() for controller wrappers so each type is stored in the correct lookup map.
• Treat InstanceWrapper objects as Nest container internals; obtain them from framework discovery/container flows rather than constructing them manually in application code.
• Use stable string or Symbol metadata keys consistently for registration and lookup.
• Expect lookup methods to return a Set<InstanceWrapper> and handle an empty set when no matching hosts were discovered.

Relationships

• IMPORTS → Type