There are data consistency issues when modifying more number of elements in a collection Vs less number of elements.
Following is a detailed explanation about the issue with example:
- Entity A has a collection of Entities AItems with cascade ALL.
- Test case :
Clear all the data inside tables representing Entity A and AItems.
Create 3 entity managers em1,em2 and em3.
create A on em1 with id "1"
add 10 elements of AItems (id's from 0-9) to the created A(id 1).
merge A ( created in the previous step)
Remove 3 elements of AItems from the merged A.
Add 3 elements of AItems ( id's 10,11,12) to the merged A (id 1).
With out committing em1
query database to fetch A and construct object result2 of entity A.
Add 3 elements of AItems ( id's 13,14,15) to fetched A ( result2)
query database to check the size of AItems that are related to A ( id 1)
The result on em3's query for AItems related to A, returns 13 as expected.
13 ( Initial 10 - em1's 3 + em1's 3 + em2's 3).
When the same test case is repeated with removing and adding 10 elements instead of 3 as before then I get wrong results.
Add initial 10 AItems (id's 0-9) for A.
em1 will remove 10 AItems from the collection of A.
em1 will add 10 AItems (id's 10-19) to collection of A.
em2 will add 10 AItems (id's 20-29) to collection of A.
Then instead of 20 elements ( Initial 10 - em1's 10 + em1's 10 + em2's 10), I see only 10 elements.
The 10 elements that I see are from em1's added AItems ( id's 10-19).
I think the cause of the issue is that, when more number of elements (compared to initial element count of collection) in a collection are modified then collection tracking is disabled and openJPA tries to do the following:
– Delete every thing from the collection
– Insert data back to collection.
While Inserting the data back it does not consider adding the dirty records ( em2's 10 added elements ) because the collection tracking is disabled.