Why "Error in mru internal routine..." ?

Question

Hello !
I have two tables SITES( site_pk, site_name,...) and SITE_DOKU( doku_pk, fk_site_pk, doku_det1, doku_det2, doku_date,...).
The primary keys are set by before insert triggers which retrieve NEXTVAL from a sequence.
Not for every SITE there is a SITE_DOKU record, but if one exists, no other for this site can be created.
I created a view SITE_VIEW as an outer join of the above tables:
CREATE OR REPLAVE VIEW SITE_VIEW AS
SELECT S.site_pk, S.site_name,
       D.doku_pk, D.doku_det1, D.doku_det2,...
  FROM SITES LEFT OUTER JOIN SITE_DOKU
         ON D.site_pk = D.fk_site_pk
/For insert, update and delete I created INSTEAD OF triggers which call procedures from a package.
In my APEX-application I created a tabular form based on this view.
The wizard asked for a primary key and I descided to take doku_pk.
I thought it's not really of interest, because handling of insert and update is managed by my instead of triggers.
To be on secure side in the instead of update trigger I tried an update and if it would fail ( %ROWCOUNT attribute = 0 ) I did an insert.
Two scenarios for my application:
A) There are two records displayed in my form:
SITE_PK  SITE_NAME  DOKU_PK  DOKU_DET1  DOKU_DET2
101      Sitename1
102      Sitename2  1001     valuedet1This means: for site 101 there doesn't exist a site_doku record, for site 102 there is site_doku record 1001
And now the behaviour of the tabular form: if I fill in a value in item DOKU_DET1 for the first line (it is therefore an insert into SITE_DOKU) or try to update the second line I get a checksum error:
"Error in mru internal routine: ORA-20001: Error in MRU: row= 1, ORA-20001: ORA-20001: Current version of data in database has changed since user initiated update process. current checksum =..."
B) There are two records displayed in my form:
SITE_PK  SITE_NAME  DOKU_PK  DOKU_DET1  DOKU_DET2
201      Sitename1  2001     valuedet1
202      Sitename2This means: for site 202 there doesn't exist a site_doku record, for site 201 there is site_doku record 2001
And now the behaviour of the tabular form: if I fill in a value in item DOKU_DET1 for the second line (it is therefore an insert into SITE_DOKU) or try to update the first line everything works well: there is an update or insert into SITE_DOKU !
Can anyone please exactly describe how APEX is handling the two not so different situations, in the first case resulting in an error, and in the second case working as I expect ?
BTW: I found a workaround (or solution) in changing the primary key definition for the view to SITE_PK. Everything is working now, but I want to understand how APEX is working !
Regards,
Heinz

Answer 1

Heinz,
There are a number of things happening when rendering and saving tabular forms. While rendering the form, the data is read from the database and shown in your tabular form. There are also checksum values built for each row by concatenating the values of your editable columns and building a checksum on the resulting string.
Then, after you entered your data and clicked the apply changes button, for each row the engine checks if there has been a change, and if so, the engine retrieves the current data for each row from the database, builds a checksum on that data and compares the checksum value with the checksum value that was computed when rendering the form. If the checksums match, the engine performs the update otherwise it raises an MRU error.
All of this is done based on your PK. And if the PK is NULL, then the engine assumes this is a new row and attempts an insert. In your case, I think your first scenario fails because the column you chose as PK is NULL, so the engine can't identify the correct row in the database. With your work-around / solution, using a different column as PK, you basically ensure that each records can be properly identified.
Hope this helps,
Regards,
Marc