问题
I have a legacy database (Cobol files actually) that I am accessing using a proprietary JDBC driver with Hibernate/JPA.
The Entity has a composite primary key with 2 columns: CODE and SITE.
In the legacy data there are records for the same CODE that can have either a specific value for SITE, or there can be a record with NULL in the SITE column which represents 'All Sites'. The theory of this file is, if you cannot find the CODE for your specific SITE then you lookup the record with NULL in the SITE (the 'catch-all').
I cannot change the structure of this 'table' as it would involve rewriting large parts of the legacy Cobol system which we don't want to do. I also cannot create views of the data.
Now when I do an em.find with the primary composite key class containing a specific code and a null for site, then Hibernate correctly finds the matching record with the NULL value in the column - All good!
But if I try to do a query using em.createQuery similar to the following:
SELECT x FROM TareWeight x WHERE x.pk.code = 'LC2'
for which there are 2 records, it returns a null object in the resulting list for the record with the NULL in the SITE column.
If I take the SQL that Hibernate uses for this query, then the 'database' returns two records, one with the NULL site and one with a specific site. It seems that when Hibernate loads the Entities from these results, it is mapping it to a null Entity object.
So either Hibernate supports it or it doesn't. Why would the em.find work but not the em.createQuery?
I know that this question is similar, but the answers seem to suggest that it is impossible. But clearly Hibernate can do the find correctly, so why does the query not work?
EDIT: OK, so I found a NullableStringType class definition on this Hibernate JIRA Issue and added it to my project.
If I add a @Type definition on the site column of the PK using this type class, then I can successfully get non-null Entities back from the SELECT query, with the site field containing whatever String text I define as the representation of null.
However, it is still behaving differently. The find returns an Entity with the site field containing null, but the query returns an Entity with the site field containing "NaN" (the default representation of null).
It still feels like these should behave the same.
UPDATE 2: Some of you want to know specifics about the 'database' in question.
It is the Genesis RDBMS engine, written by Trifox Inc.. The data is stored in AcuCobol (now Micro Focus) Vision indexed files.
We have the configuration set to translate blank (SPACES) alphanumeric fields to NULL, hence our file records which contain spaces for the PK field are being translated to NULL. I can specifically select the appropriate record by using WHERE site_id IS NULL, so the RDBMS is treating these blank fields as an SQL NULL.
Having said all that I do not believe that this issue has anything to do with the 'database', apart from the fact that it is unusual to have PK fields being null.
What's more, if I log the SQL that Hibernate is using for both the find and the query, they are almost identical.
Here's the SQL for the find:
select tareweight0_.CODE as CODE274_0_, tareweight0_.SITE as SITE274_0_,
tareweight0_.WEIGHT as WEIGHT274_0_ from TARE_WEIGHT tareweight0_
where tareweight0_.CODE=? and tareweight0_.SITE=?
And here's the SQL for the Query:
select tareweight0_.CODE as CODE274_, tareweight0_.SITE as SITE274_,
tareweight0_.WEIGHT as WEIGHT274_ from TARE_WEIGHT tareweight0_
where tareweight0_.CODE=? and tareweight0_.SITE=?
As you can see, the only difference is the column alias names.
UPDATE 3: Here's some example data:
select code, site, weight from tare_weight where code like 'LC%';
CODE SITE WEIGHT
------ ------ -------
LC1 .81
LC2 .83
LC2 BEENLH .81
LC3 1.07
LC3 BEENLH 1.05
LC4 1.05
LCH1 .91
LCH2 .93
LCH2 BEENLH .91
LCH6 1.13
LCH6 BEENLH 1.11
And searching specifically for NULL:
select code, site, weight from tare_weight where code like 'LC%' and site IS NULL;
CODE SITE WEIGHT
------ ------ -------
LC1 .81
LC2 .83
LC3 1.07
LC4 1.05
LCH1 .91
LCH2 .93
LCH6 1.13
回答1:
"So either they support it or they don't"
TL;DR That expectation/feeling is unjustified. The unsupported functionality in your link (& mine below) is exactly yours. "Not supporting" it means that if you do it then Hibernate can do anything they want. You are lucky that they (seem to) return reasonable values. (Although it's just a guess how they are acting. You don't have a specification.) There is no reason to expect anything, let alone consistency. When behaviour is just a consequence of some unsupported case arising, any "why" is most likely just an artifact of how the code was written with other cases in mind.
Here is a(n old) support thread answered by the Hibernate Team:
Post subject: Composite key with null value for one column
PostPosted: Mon Jul 03, 2006 2:21 am
I have table with composite primary key and I created following mapping for the table.As it is possible to insert a null value for any column in composite key as long as the combination of all columns is Unique, I have record in teh table which has null value for V_CHAR2 column ( which is part of composite key ) . when I execute a query on this entity I get null values for the records which are having null value of V_CHAR2 column. What's wrong in my mapping and implementation..
Posted: Tue Jul 11, 2006 9:09 am
Hibernate Team
a primary key cannot be null (neither fully or partial)
Posted: Sat Jan 06, 2007 5:35 am
Hibernate Team
sorry to disapoint you but null in primary key is not supported - primarily because doing join's and comparisons will require alot of painfullly stupid code that is just not needed anywhere else.....think about it and you will see (e.g. how do you do a correct join with such a table)
This is not surprising, because NULL is not allowed in a PK column in SQL. A PRIMARY KEY declaration is a synonym for UNIQUE NOT NULL. NULL is not equal to anything with the (misguided) intent that some unrecorded value is not known to be equal. (Your expectations of some kind of exception for at least some occasions of NULL in a PK equaling a NULL in a condition is contrary to SQL.) Given that NULL is not allowed in PK values, we can expect PK optimizations related to 1:1 mappings and to sets rather than bags of rows to assume there are no NULLs when it's convenient. One can expect that Hibernate decided to not worry about what their implementation did with cases that shouldn't arise in the SQL. Too bad they don't tell you that on compilation or execution. Hopefully it is in documentation.)
Even find differing from createQueryre NULL is not surprising. The former involves one value while the latter involves what are expected to be sets (not bags) of rows without NULLs (but aren't).
A workaround may be to not treat any column of a primary key as NULL but as the actual string of spaces in storage. (Whatever this means given your storage/DBMS/hibernate/JPA/Java stack. You haven't given us enough information to know whether your Cobol view of the database would be impeded by not mapping spaces to NULL for your JPA). With your data you can still declare a UNIQUE index on the columns.
来源：https://stackoverflow.com/questions/34079524/jpa-query-returning-nulls-composite-key-with-null-column