Does anyone fancy exercising their little grey cells on this fine
Monday?
I have an entity with relevant fields declared as:
@PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable = "true")
public class Thing implements Serializable
{
private static final long serialVersionUID = 1L;
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
@Extension(vendorName="datanucleus", key="gae.encoded-pk",
value="true")
private String sEncodedKey;
@Persistent
@Extension(vendorName="datanucleus", key="gae.pk-id", value="true")
private Long loID;
@Persistent
private Date dtCreated;
}
I want to display pages of my persistent entity to the user, which
each page having, say, 20 items displayed. I want to use the class
JDOCursorHelper (and stick with JDO if possible). The GAE/J
documentation shows me how I can paginate forwards:
· Query the first 21 entities and store in an ArrayList<Thing>.
· If 21 are returned, set a "there are more entities" flag and
discard the 21st entity from the list, otherwise reset the flag.
· Show the 20 entities.
· Store the cursor as a web-safe string.
· If the user hits "Next" (enabled if the flag is set) then:
· retrieve the cursor from the web-safe string.
· get the next 21 entities
· etc.
The fun starts if I want to allow the user to paginate the previous
pages. One method that I can think of stores the web-safe string of
each cursor in a list kept in the user's session object. As "Next" is
hit, the cursor just generated can be pushed onto the list. To
traverse backwards, I pop (delete) the latest cursor string from the
session's list and then use the previous one (now the latest) to
produce the previous page's items.
Can anyone think of a smarter way to paginate backwards without
storing a list of web-safe cursor strings in a user's session?
(I could generate an additional persistent field which holds the
concatenated string values
[dtCreated to milliseconds using Calendar] + "|" + sEncodedKey
but the GAE/J article for this says that this way is no longer
recommended.)
Regards,
Ian
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