Synopsis

Describes how to get the most current values for an object by either refreshing during a fetch or invalidating an object.

Discussion

Enterprise Objects Framework (EOF) automatically caches the raw dictionary of values and unique objects that are fetched from the database. The raw dictionary is cached in a set of snapshots based on a GlobalID derived from the primary keys of the database rows. All peer EOEditingContexts share this common cache of snapshots. Each EOEditingContext stores its own unique object instance for a given GlobalID. Whenever the common snapshots change, an EOObjectsChangedInStore notification is posted and all associated EOEditingContexts attempt to reconcile themselves with the new snapshot.

When an EOEditingContext fetches a row, it creates one unique object within the EditingContext based on the GlobalID. This "uniquing" process enables EOF to return the same object during a future fetch. Uniquing is an important part of EOF, because having multiple instances of the same object in the same editing context is not desirable.

When a peer editing context such as another session's defaultEditingContext fires a fault to a GlobalID that has already been fetched, the last fetched data (in the snapshot cache) is used as the source instead of a real database fetch. A new object is then instantiated and uniqued in this peer editing context.

The default behavior for fetching and uniquing an object involves comparing a newly fetched row's primary keys to the previously fetched snapshot. If a snapshot already exists with the same primary keys, EOF will completely ignore the fetched row and just use the previous snapshot.

This appears to be undesirable since the new fetch might have updated data. However, if the default was to always update the objects to the latest fetched data, your objects might be updated at unknown and invalid times.

The following are two ways that an instantiated object can be refreshed with the latest database values:

• The object (row) is returned from a new fetch that specifically requests refreshing.
• The editing context for the object is asked to invalidate the object, which releases the snapshot for this object and turns all instances back to a faults.

 
EOEditingContext ec=session().defaultEditingContext(); 
EOFetchSpecification fs=new EOFetchSpecification 
    ("Movie", qual, null); 
fs.setRefreshesRefetchedObjects(true); 
movies=ec.objectsWithFetchSpecification(fs);

 
EOEditingContext *ec=[[self session] defaultEditingContext]; 
EOFetchSpecification *fs=[EOFetchSpecification 
    fetchSpecificationWithEntityName: @"Movie"  
    qualifier:nil sortOrderings: nil]; 
[fs setRefreshesRefetchedObjects:YES]; 
movies =[ec objectsWithFetchSpecification: fs];

See Also

• Invalidating Objects
• EOEditingContext class specification in the Enterprise Objects Framework Reference
• EOFetchSpecification class specification in the Enterprise Objects Framework Reference

Questions

• How do I force a fetch and update my existing objects?
• Why does my object still have old values even after a refetch?
• How can I force an object to update its values?

Keywords

• Refetch
• Invalidate
• Update
• Snapshot

Revision History

23 July, 1998. David Scheck. First Draft.

19 November, 1998. Clif Liu. Second Draft.