AppEngine datastore query for all entities that have a given property (Java)

问题

I am trying to figure out an elegant way to query all of the entities in an AppEngine datastore that have a certain property. Since entities that lack a property aren't included in an index, basically what I want to do is to retrieve the index for a given property. I'm sure it's possible to do something like:

Filter bigger = new FilterPredicate(PROPERTY,
                  FilterOperator.GREATER_THAN_OR_EQUAL,
                  0);

Filter smaller = new FilterPredicate(PROPERTY,
                  FilterOperator.LESS_THAN_OR_EQUAL,
                  0);

Filter present = CompositeFilterOperator.or(bigger, smaller);

Query q = new Query(KIND).setFilter(present);

but it doesn't look like a very elegant (or efficient) solution. Does anyone have a better idea?

回答1:

If you don't need entities with null values, you can use this:

Filter filter = new FilterPredicate(PROPERTY, FilterOperator.NOT_EQUAL, null);

It may look simpler, but NOT_EQUAL filter actually results in two separate queries - just as your solution, unless App Engine is smart about null values - that I don't know.

回答2:

The property I am interested in is actually a list which may have lots of values, so I decided to leave it unindexed, and add a separate boolean property HAS_LIST. That allows me to do a single query for results at the cost of a slightly bigger entity. Still not a very elegant solution, but perhaps a little more efficient.

来源：https://stackoverflow.com/questions/22199729/appengine-datastore-query-for-all-entities-that-have-a-given-property-java