Nestes Schema virtual

I am trying to access the products collection in my find method. I added a .virtual method in order to access it but I currently get following error messsage:

MissingSchemaError: Schema hasn't been registered for model "ProductDocument".

So far my implementation:

export type PriceDocument = Price & Document;

@Schema({ timestamps: true })
export class Price {
  @Prop({
    ref: 'ProductDocument',
    required: true,
  })
  uid: string;

  @Prop()
  standardPrice: number;

  @Prop({ required: true })
  startDate: Date;

  @Prop()
  endDate: Date;
}

export const PriceSchema = SchemaFactory.createForClass<Price>(Price)
  .index({
    uid: 1, startDate: 1, endDate: 1,
  }, { unique: true });

PriceSchema.virtual('products', {
  ref: 'ProductDocument',
  localField: 'uid',
  foreignField: 'uid',
});

ProductPriceSchema.set('toObject', { virtuals: true });
ProductPriceSchema.set('toJSON', { virtuals: true });

This is how I try to access it in my find method

    const productPriceDocument = await this.productPrice.find({
      $and: queries, // dynamic queries
    }).populate('products').exec();