如何避免插入到MongoDB的重复条目

Question

在向DB输入新的DBObject时，我只想说明它确实是唯一的，而且Collection不包含密钥字段副本。

现在的情况如下：

public abstract class AbstractMongoDAO<ID, MODEL> implements GenericDAO<ID, MODEL> {

    protected Mongo client;

    protected Class<MODEL> model;

    protected DBCollection dbCollection;

    /**
     * Contains model data : unique key name and name of get method
     */
    protected KeyField keyField;

    @SuppressWarnings("unchecked")
    protected AbstractMongoDAO() {
        ParameterizedType genericSuperclass = (ParameterizedType) this.getClass().getGenericSuperclass();
        model = (Class<MODEL>) genericSuperclass.getActualTypeArguments()[1];
        getKeyField();
    }

    public void connect() throws UnknownHostException {
        client = new MongoClient(Config.getMongoHost(), Integer.parseInt(Config.getMongoPort()));
        DB clientDB = client.getDB(Config.getMongoDb());
        clientDB.authenticate(Config.getMongoDbUser(), Config.getMongoDbPass().toCharArray());
        dbCollection = clientDB.getCollection(getCollectionName(model));
    }

    public void disconnect() {
        if (client != null) {
            client.close();
        }
    }

    @Override
    public void create(MODEL model) {
        Object keyValue = get(model);
        try {
            ObjectMapper mapper = new ObjectMapper();
            String requestAsString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(model);
            // check if not presented
            BasicDBObject dbObject = new BasicDBObject((String) keyValue, requestAsString);
            dbCollection.ensureIndex(dbObject, new BasicDBObject("unique", true));
            dbCollection.insert(new BasicDBObject((String) keyValue, requestAsString));
        } catch (Throwable e) {
            throw new RuntimeException(String.format("Duplicate parameters '%s' : '%s'", keyField.id(), keyValue));
        }
    }

private Object get(MODEL model) {
    Object result = null;
    try {
        Method m = this.model.getMethod(this.keyField.get());
        result = m.invoke(model);
    } catch (Exception e) {
        throw new RuntimeException(String.format("Couldn't find method by name '%s' at class '%s'", this.keyField.get(), this.model.getName()));
    }
    return result;
}

/**
 * Extract the name of collection that is specified at '@Entity' annotation.
 *
 * @param clazz is model class object.
 * @return the name of collection that is specified.
 */
private String getCollectionName(Class<MODEL> clazz) {
    Entity entity = clazz.getAnnotation(Entity.class);
    String tableName = entity.value();
    if (tableName.equals(Mapper.IGNORED_FIELDNAME)) {
        // think about usual logger
        tableName = clazz.getName();
    }
    return tableName;
}

private void getKeyField() {
    for (Field field : this.model.getDeclaredFields()) {
        if (field.isAnnotationPresent(KeyField.class)) {
            keyField = field.getAnnotation(KeyField.class);
            break;
        }
    }
    if (keyField == null) {
        throw new RuntimeException(String.format("Couldn't find key field at class : '%s'", model.getName()));
    }
}

KeyFeld是自定义注释：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface KeyField {

    String id();

    String get();

    String statusProp() default "ALL";

但我并不认为这个解决方案能真正证明这一点。我刚来蒙戈。

有什么建议吗？

Answer 1

在MonboDb中，可以使用_id字段维护唯一性。如果我们不提供该字段的值，MongoDB将自动为该粒子集合创建一个唯一的id。

因此，在您的示例中，只需在java中创建一个名为_id的属性&在这里分配您的唯一字段值。如果复制，它将引发异常。

Answer 2

使用Spring (问题是用spring-data标记的，这就是我建议它的原因)，您所需要的就是：

 // Your types

 class YourType {

    BigInteger id;
    @Indexed(unique = true) String emailAddress;
    …
 }

 interface YourTypeRepository extends CrudRepository<YourType, BigInteger> { }

 // Infrastructure setup, if you use Spring as container prefer @EnableMongoRepositories

 MongoOperations operations = new MongoTemplate(new MongoClient(), "myDatabase");
 MongoRepositoryFactory factory = new MongoRepositoryFactory(operations);
 YourTypeRepository repository = factory.getRepository(YourTypeRepository.class);

 // Now use it…

 YourType first = …; // set email address
 YourType second = …; // set same email address

 repository.save(first);
 repository.save(second); // will throw an exception

与原始问题最相关的关键部分是@Indexed，因为这将导致在创建存储库时创建所需的唯一索引。

你所能超越的是：

• 无需手动实现任何存储库(已删除的代码不包含bug \o/)
• 对象到文档的自动转换
• 自动索引创建
• 强大的存储库抽象，可以通过声明查询方法轻松查询数据

有关更多细节，请查看参考文献。