How do you use list properties in Google App Engin

问题:

An object to be placed in the datastore will have a set of tags.

public class Model 
{
    List<String> tagList
    ...
}

In Python, the Google App Engine has the notion of list properties. What is the equivalent notion in Java (if it exists) and how would you use list properties in Java, in JPA and/or in JDO?

回答1:

See my blog post exactly on this: Efficient Keyword Search with Relation Index Entities and Objectify for Google Datastore. It talks about implementing search with list properties using Relation Index Entities and Objectify.

To summarize:

  Query<DocumentKeywords> query = ofy.query(DocumentKeywords.class);
  for (String keyword : keywords) {
    query = query.filter("keywords", keyword);
  }

  Set<Key<Document>> keys = query.<Document>fetchParentKeys();

  Collection<Document> documents = ofy.get(keys).values();

where DocumentKeywords contains a list property (collection) of all keywords for its Document entity, and Document entity is a parent for DocumentKeywords.

回答2:

In JDO use

@Persistent
private List<ContactInfo> contactInfoSets;