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Template Resolver

Load email templates from external sources like databases, S3, or APIs instead of the filesystem.

Interface

Implement the TemplateResolver interface:

import { TemplateResolver, ResolvedTemplate } from '@nestjs-modules/mailer';

export class DatabaseTemplateResolver implements TemplateResolver {
  constructor(private readonly templateRepo: TemplateRepository) {}

  async resolve(
    templateName: string,
    context?: Record<string, any>,
  ): Promise<ResolvedTemplate> {
    const template = await this.templateRepo.findByName(templateName);
    return {
      content: template.html,
      metadata: {
        subject: template.subject,
      },
    };
  }
}

Configuration

Pass the resolver in your module config:

MailerModule.forRoot({
  transport: { host: 'smtp.example.com', port: 587 },
  template: {
    adapter: new HandlebarsAdapter(),
    resolver: new DatabaseTemplateResolver(templateRepo),
  },
})

How It Works

1. When sendMail() is called with a template name
2. If a resolver is configured and no html is provided
3. The resolver is called to fetch the template content
4. The resolved content is set as html on the mail options
5. If the resolved template includes metadata.subject and no subject was provided, it is applied automatically

Example: S3 Resolver

import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3';

export class S3TemplateResolver implements TemplateResolver {
  private s3 = new S3Client({ region: 'us-east-1' });

  async resolve(templateName: string): Promise<ResolvedTemplate> {
    const command = new GetObjectCommand({
      Bucket: 'my-email-templates',
      Key: `${templateName}.html`,
    });

    const response = await this.s3.send(command);
    const content = await response.Body!.transformToString();

    return { content };
  }
}

Example: API Resolver

export class ApiTemplateResolver implements TemplateResolver {
  constructor(private readonly httpService: HttpService) {}

  async resolve(
    templateName: string,
    context?: Record<string, any>,
  ): Promise<ResolvedTemplate> {
    const { data } = await firstValueFrom(
      this.httpService.get(`https://templates.internal/api/${templateName}`),
    );

    return {
      content: data.html,
      metadata: { subject: data.subject },
    };
  }
}

INFO

When a template resolver is used, the resolved content bypasses the file-system lookup of template adapters. The adapter's compilation step (variable interpolation, partials, etc.) still applies if you use html with template syntax.