Merge needlessly/dangerously copies immutable fields.

[jamoore]

Was: Passing non-proxied one-to-manys to server can return empty (non-null) collection (See comments below)

Several model objects have methods for adding items to collection-valued fields:

 * Pixels.addThumbnail(Thumbnail)
 * Project.linkDataset(Dataset)

These calls add the argument to their own collection and then call the reverse method on the arguments:

 * Thumnail.setPixels(Pixels)
 * ProjectDatasetLink.setProject(Project)

The collections are ignored by Hibernate, and only the reverse methods have an effect. This is known behavior.

However, when one uses only the reverse methods (e.g. setPixels(Pixels)) and the Pixels instance is not a proxy (e.g. pixels.isLoaded()==true) then the Pixels instance which is returned by the IUpdate.saveAndReturnObject() method is neither unloaded, nor is the collection nulled, nor does it contain the newly added Thumbnail which could easily lead to data corruption.

[jmoore]

r926 has the failing tests.

[jmoore]

#350 blocks the fix for this ticket.

[jmoore]

r931 provides a partial fix.

[jmoore]

r940 removes debugging.

[jmoore]

r942 reverts the patrial fix. Strange Hibernate bugs arose which couldn't easily be tracked down.

[jmoore]

r955 has another non-collection-valued example of this. Specifically, that if a user attempts to set the creation event to some invalid value (any other value is invalid since creation event is immutable) for an IObject, though the database is unaffected, the value returned to the user will be the invalid one.

[jmoore]

Hibernate Docs 10.11:
"It doesn't usually make sense to enable cascade on a <many-to-one> or <many-to-many> association. Cascade is often useful for <one-to-one> and <one-to-many> associations."

This may be the cause of our problem. Have to think about what the repercussions are of turning off cascades on many-to-one in our scenario. (Along the lines of always having to pass in the one-to-many side of relationships (e.g. Image for Image<->Pixels to have everything saved)

[jmoore]

So, this has nothing to do with cascading. It actually is a mirkier question of the specification of "merge()". What's happening is thus:

• Object A (detached on client) has field B set to C.
• Field B, however, is immutable and won't be set in SQL or dirty detection. (i.e. no errors)
• Session.merge(), however, copies all values from detached A to persistent A, which gets returned to user.

Working on a workaround.

[jmoore]

Moving to milestone:Blocked. Hibernate does not seem provide access for me to write our workaround.

See r990 for the workaround. (A patch for MergeEventListener? is also necessary)

[jamoore]

Unless profiling shows this to be a considerable issue, we won't be working on this.