I have a question about hibernate constrains, about one issue that I never had before.
Imagine that I have a table(Snapshot) where I can add some snapshot rows, which every single one have to be related with just one row of another tables. But this relationship is not only with one table, multiple tables can join with this snapshot table. But I want to prevent that once one row of snapshot is already link with another row table let´s say:
A.row1->Snapshot.row1
It´s not possible that another table pick up the same row for his relationship
B.row1->Snapshot.row1.
Because if dont, imagine the issue when I´m trying to do a delete on cascade on A.
Any idea how to make this work with hibernate unique constraints
in Snapshot make the field that links to the other table (i suppose it's called row1) unique.
#Column(unique = true)
edit:
you can not control how many other tables are using your primary key. what you can do is introduce a new table where you manage your linking. there you coudl have to columns one called link_from and the other one link_to and make link_to unique.
Related
I am trying to have a table with an "order" column to allow rearranging the order of data. Is this possible using jpa? Maybe something similar to #OrderColumn but on the table itself.
Basically I want to add a new column called "order" that saves the order the records. If a record is added, it would automatically get a "order" value. If a record was deleted, the "order" of the remaining would be automatically updated. Additionally if possible, to rearrange the orders by moving one record to an lower "order" and it would push the others
There is no way to do this out of the box, but you can implement this yourself if you want. Just query for the count of objects right before persisting and set the count + 1 as value for that order column. Make sure that the order column is declared as being unique i.e. with a unique constraint.
Note that your requirement is pretty exotic and will likely require some kind of table lock or retry mechanism if you have high concurrency.
IMO you should ask whoever gave you this requirement what the goal is that should be achieved. I bet that you will find out you don't need this after all.
I'm working on a Java project which needs to be able to alter all the primary keys in a table - and in most cases - some other values in the row as well.
The problem I have is that, if I update a row by selecting by its old primary key (SET pk=new_pk WHERE pk=old_pk) I get duplicate rows (since the old PK value may be equal to another row's new PK value and both rows are then updated).
I figured that in Oracle and some other DBs I might be able to do this with ROWNUM or something similar, but the system should work with most DB systems and right now we can't get this to work for MySQL.
I should add that I don't have access to change the schema of the DB - so, I can't add a column.
What I have tried:
Updating ResultSets directly with RS.updateRow() - this seems to
work, but is very slow.
Hashing the PK's in the table, storing the hash in code and selecting on the hashed PK. This acts sort of as a signature, since a
hashed PK indicates that the row has been read but not yet updated
and I can avoid appropriate rows that way. The issue with this seems
to have been hash collisions as I was getting duplicate PKs.
PS:
I realise this sounds like either a terrible idea, or terrible design, but we really have no choice. The system I'm working on aims to anonymize private data and that may entail changing PKs in some tables. Don't fear, we do account for FK references.
In this case you can use simple update with delta = max Pk from updating table
select delta
select max(pk) as delta from table
and then use it in query
update table SET pk=pk+delta+1
Before this operation you need to disable constraints. And don't forget that you should also update foreign keys.
I have two tables, let call it table A and B. Table A has a foreign key of table B. My system creates first a row in table B, and in another screen user can create a row in table A that is related to the created row in table B.
These two rows need be send to a specific SymmetricDS client, to do this I'm using a subselect router for each table. The problem is: the row created in table B only know where it need go when the row in table A is created. When it happens, SymmetricDS already evaluated the subselect router of table B and considered the batch as unrouted. As the row of table B was not routed, the client can't create the row in table A due a foreign key error.
Is there a way I can guarantee that the two rows will synchronize together?
yes there is. use trigger customization. you'll have to wait until the version 3.7 is released or take the latest version of the source, apply the patch http://www.symmetricds.org/issues/view.php?id=1570 and declare before trigger customization for the table A which will update the row with the foreign key in the table B and have it being routed to the target before the row in table A
i´m currently working on my first Java application based on a MySQL DB. I´m using EclipseLink2.0 and NetBeans, at the time i am facing a behaviour i cannot explain, maybe someone has stumbled over this problem in the past and can help me with this. Here goes:
Table 1 has the PK of table 2 as Fk. Application-side, there is an UI where users can generate content for table 1. The value for the fk(Table2ID) is beeing chosen with a dropdown menu, which gets each values by reading the Collection of table2 rows. Now, when i try to change the value for the fk to another (already existing) value, instead of doing just that, a new row with a fresh ID is generated on table2, all other column values are cloned from the row i tried to change the Fk to. So, for example, when i try to set table1 rows 3,4 and 5 to table1.fkcolumn =6 (i.e Table2ID=6), the program instead clones the row with ID=6 3 times and sets each of the table1 columns to one of them.
Any help would be greatly appreciated .
The problem is you are changing the primary key of an entity. In EclipseLink, when you change the PK of an entity, you have a new entity. As such, Eclipselink inserts the new rows and leaves the old rows alone.
To get around this you have three choices:
1) Change the database. Primary keys really shouldn't be changed.
2) Set the application to execute an update query which changes the primary key values and requery them.
3) Delete the old rows and re-create with a new primary key.
I am little bit newbie in Hibernate and DBMS and stuck on a problem. I have two table. One is A and second is B. There is Many-to-One relationship between these two tables. Now on removing one record from table A I do not want to delete the record from table B. What type of cascading strategy should I apply.
Please help.
Here's what Hibernate manual says.
Based on what you wrote I think you should use cascade="persist,merge,save-update"