Insert embeded document without reading whole document - spring, mongo

You should use MongoTemplate to update product with push to add to existing products. Something like

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.mapping.Document;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.data.mongodb.core.query.Query;
import org.springframework.data.mongodb.core.query.Update;

import java.util.List;

@SpringBootApplication
public class So62173077Application {

    public static void main(String[] args) {
        SpringApplication.run(So62173077Application.class, args);
    }

    @Autowired
    private MongoTemplate mongoTemplate;

    @Document(collection = "factory")
    public class Factory
    {
        private Long id;
        private List<Product> products;

    }

    public Long createFactory() {
        Factory factory = new Factory();
        factory.id = 1L;
        return mongoTemplate.insert(factory).id;
    }

    public void addProduct(Long id) {
        Query query = new Query();
        query.addCriteria(Criteria.where("id").is(id));
        Update update = new Update();
        Product product = new Product();
        product.name = "stackoverflow";
        update.push("products", product);
        mongoTemplate.updateFirst(query, update, Factory.class);
    }

    private class Product {
        private String name;
    }

    @Bean
    public ApplicationRunner runner() {
        return args -> {
            //Long id = createFactory();
            addProduct(1L);

        };
    }

}