覆盖Google App Engine- Java中的数据存储区实体

Question

I have an application in which I want to overwrite an individual entity. This is how I originally create entity logs:

Entity log = new Entity("Log", "Logkey");
    String property1 = req.getParameter("property1");
    String property2 = req.getParameter("property2");
    log.setProperty("property1", property1);
    log.setProperty("property2", property2);
    datastore.put(log);

Here is how entity logs are retrieved to be overwritten:

Query query = new Query("Log", "Logkey")
             .setFilter(timeStampFilter);
List<Entity> logs = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(1));  
request.setAttribute("logs", logs);

and sent to a jsp form page as value="${log.properties.property1}" where they should be overwritten. This entry is then sent to a second servlet with the POST method and requested as parameters as in the earlier code but saved as a new entity with the same kind:

Entity edit_log = new Entity("Log", "Logkey");
        String property1 = req.getParameter("property1");
        String property2 = req.getParameter("property2");
        edit_log.setProperty("property1", property1);

Answer 1

For rewriting and existing entity, after retreiving a specific log by timestamp, you can get the key of this log using getKey() method and then create an entity with this key and the new details. Now when you put this new entity to the datastore it will replace the earlier one with the same key

Answer 2

With the code you've written, you only have a single Log entity in your datastore with the key "Logkey" that you are constantly overwriting.

If you're using some other code to retrieve entities and rewrite them, then you'll need to show that other code. Otherwise, this question is poorly written, because the code given is already describing what you want to do (always overwrite the same entity).

If you have code elsewhere creating/saving entities, it's best to show that too.

Edit : It looks like you end up creating a nested entity with the data from the old entity in a new entity with the same key. It's far easier just to reuse the entity you received from the query.

log = logs.get(0)
log.setProperty("property1", req.getParameter("property1");
log.setProperty("property2", req.getParameter("property2");
datastore.put(log);

Furthermore, since you actually know the key ("Logkey"), you don't need to issue a datastore query, you can just fetch the entity by key - which is good because you get around eventual-consistency behavior.

Answer 3

如果新实体的密钥与原始实体的密钥相同，则在存储它时它将覆盖旧实体。