Tag: datastore

Searching the datastore using date-based entity properties can often be confusing to new App Engine developers. Here’s how to search for all recently created entities.

First, each entity has to record the time it was created. Add the following line of code when constructing your entities (entity represents the Entity object being created):

entity.setProperty("add_date", new Date());//Records the date this entity was added.

Here’s the code for the query. This code example searches for all entities created within the last 24 hours with a kind of mail. The resulting entities are placed into the variable results (which is a standard java.util.List object).

/**
 * Creates a Date object that represents a point in time 
 * 24 hours ago.
 */
Date current_date = new Date();//The current date.
long current_date_in_ms = current_date.getTime();//The current date in milliseconds.
//Take the current date in milliseconds, and subtract 24 hours worth 
//of milliseconds (1000 milliseconds per second, times 60 seconds per 
//minute, times 60 minutes per hour, times 24 hours in a day).
long one_day_ago_in_ms = current_date_in_ms - (1000 * 60 * 60 * 24);
Date one_day_ago = new Date(one_day_ago_in_ms);//Create a new Date object representing one day ago.
//Search all entities with the kind "mail". 
//We create a Filter that requires that the add_date property be greater 
//than the Date we computed above.
Query q = new Query("mail");
Query.Filter date_filter = new Query.FilterPredicate("add_date", Query.FilterOperator.GREATER_THAN_OR_EQUAL, one_day_ago);
q.setFilter(date_filter);
PreparedQuery pq = DatastoreServiceFactory.getDatastoreService().prepare(q);
List<Entity> results = pq.asList(FetchOptions.Builder.withDefaults());

 

Storing Data

Essentially, the datastore is a highly replicated, highly reliable place to store data. However, it is not a SQL database that you may have experience with. Instead, the datastore stores Entities, which are grouped into categories called kinds. Each entity has a unique ID or name – Google App Engine can set a unique ID, or you can set a name in your application code.

To create an entity with a mail kind, we use this line (App Engine allocates a unique ID for us automatically):

Entity entity = new Entity("mail");

Entities can have properties, which are name/valid pairs. To add a property, do this:

entity.setProperty("subject", subject); //Key, then value.

Here’s a complete code example to create an entity, set properties, and store it:

Entity entity = new Entity("mail");//Creates the entity with a kind of "mail", GAE automatically allocates an ID
entity.setUnindexedProperty("from", from);
entity.setProperty("subject", subject);
entity.setProperty("add_date", new Date());//Records the date this entity was added.
DatastoreServiceFactory.getDatastoreService().put(entity);

Note the use of setUnindexedProperty. This method informs App Engine that we won’t be searching on this property, so don’t build a search index for it. This is useful to reduce the cost of storing entities.

Datastore Query

Querying the datastore is simple as well.

Here’s an example piece of code which queries for all Entities with the kind of mail, and requiring that all properties returned have the property add_date greater than the value stored in date_var :

Query q = new Query("mail");
Query.Filter query_filter = new Query.FilterPredicate("add_date", Query.FilterOperator.GREATER_THAN_OR_EQUAL, date_var);
q.setFilter(query_filter);
PreparedQuery pq = DatastoreServiceFactory.getDatastoreService().prepare(q);
List<Entity> results = pq.asList(FetchOptions.Builder.withDefaults());

From here, we can iterate through the List, retrieve each Entity, and conduct operations on each Entity:

for (int i = 0; i < results.size(); i++) {
    Entity entity = results.get(i);
    //Do anything needed with the information from each entity.
}

Deleting

And finally, here is how to delete an entity (entity is the Entity object being deleted).

Key key = entity.getKey();
DatastoreServiceFactory.getAsyncDatastoreService().delete(key);