Resolving NestJS Dependency Injection Error for Model in a Service

If you encounter an error indicating that NestJS cannot resolve a Model in a service, it’s likely due to a missing injection setup. In the service constructor, you may be attempting to inject multiple models, but one or more models might not be correctly registered or injected.

Let’s walk through the issue and how to resolve it.

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Problem Overview:

In your module, you may have registered several models, but a model might be missing from the service’s constructor injection, leading to a runtime error.

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Solution: Add @InjectModel() Decorator

To properly inject the model, ensure you use the @InjectModel() decorator in the service constructor.

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Updated Code Example:

generic.service.ts

import { Injectable } from '@nestjs/common';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { GenericEntity } from './schemas/generic-entity.schema';
import { AnotherEntity } from './schemas/another-entity.schema';

@Injectable()
export class GenericService {
  constructor(
    @InjectModel(GenericEntity.name)
    private readonly genericEntityModel: Model<GenericEntity>,
    @InjectModel(AnotherEntity.name)  // Add this to inject another model
    private readonly anotherEntityModel: Model<AnotherEntity>,
  ) {}

  // Example service function
  async getAllEntities(): Promise<GenericEntity[]> {
    return this.genericEntityModel.find().exec();
  }

  async getAnotherEntityById(id: string): Promise<AnotherEntity | null> {
    return this.anotherEntityModel.findById(id).exec();
  }
}

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Explanation:

• The @InjectModel(GenericEntity.name) and @InjectModel(AnotherEntity.name) decorators register the corresponding models within the service.

• If the model schema is registered in the module as:

  { name: GenericEntity.name, schema: GenericEntitySchema },
  { name: AnotherEntity.name, schema: AnotherEntitySchema },
  

  the service constructor must inject both models using @InjectModel().

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Other Potential Issues to Check:

1. Ensure Correct Schema Export: Verify that the schemas are properly exported from their files. For example:

   export const GenericEntitySchema = SchemaFactory.createForClass(GenericEntity);
   

2. Import Required Modules: If a service depends on models or services from other modules, ensure the module imports them:

   imports: [RelatedModule],
   

   This ensures that the necessary models and services are accessible.

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Final Thoughts:

By adding the correct @InjectModel decorators and ensuring that dependencies are properly registered and imported, you can resolve dependency injection errors in NestJS. Ensuring your module structure is consistent and complete will prevent common runtime issues.

Feel free to reach out if you need further help with debugging or NestJS module setup! 🚀