Hello,

I have 2 classes that are related through a one-to-one association, which is mapped to a foreign key. I am doing a query that retreives all instances of class A. When I look at the generated SQL, I see that for each A, another statement is issued to get the related B - leading to the infamous N+1 problem with extreme amounts of SQL being generated.

Since I do not need the A->B references in my particular case, I tried to modify the mapping such that the A->B reference would be resolved lazily. However, I seem to be unable to do away with the redundant SQL, no matter what I try. I have even found a section in the docs which seems to indicate that this is not possible at all, which I would find rather shocking, and, in my case, unacceptable.

Hibernate version: 3.1.2

mapping A:
one-to-one name="b" lazy="proxy" property-ref="a"

mapping B:
many-to-one name="a" column="a_id" update="false" unique="true" not-null="true"

Query:
from A a

thanks,
Christian

do you have constrained="false" ?

I dont have the constrained= attribute set, which leads to a "false" default value. But even setting it to true does not change anything.

additionally, the association foreign key is located on the B side, not the A one - therefore, if I understand the "constrained" thing correctly, it would be plain wrong to set it to true.

I did read the statement in the docs that stated that a constrained="false" association could not be fetched lazily. If that statement applies to my situation, then I am pretty disappointed. My relational model and mapping seem perfectly normal to me, and there really is no alternative in my case.

The restriction hibernate imposes here does not seem reasonable to me at all.

Christian

This is from reference documentation ( section 5.1.11 )



Have you tried the option lazy="no-proxy"

I have tried that, too, although it is still unclear to me what that option means resp. what the difference between proxy/no-proxy is.

The last part of the quote from the docs you give says that for one-to-one associations that have constrained="false" (which is probably the default) lazy fetchning is impossible.
As I am loading the objects on the non-FK side (property-ref attribute), I cannot specify constrained=true. And even when I do, it doesnt change things.

I cannot believe that I am stuck here..

hmm.. does this really mean that hibernate does NOT do away with the infamous N+1 problem, which plagued persistence frameworks in the early days?

I ran into the same problem here. There is n+1 selection. This is my configuration and resulting query:

In Document.hbm.xml:

<one-to-one name="estimate" class="Estimate"
constrained="true"
fetch="select"
property-ref="document"
lazy="proxy"/>


In Estimate.hbm.xml:

<many-to-one
name="document"
column="DOCUMENT_ID"
class="Document"
unique="true"
not-null="true"
>
</many-to-one>


findDocumentByDescription:
<query name = "findDocumentByDescription">
<![CDATA[
select d from Document d
where
d.description='One-2-One test'
]]>
</query>
generate queries like this:

Hibernate: select document0_.DOCUMENT_ID as DOCUMENT1_, document0_.TCN as TCN0_, document0_.CO_CD as CO3_0_, document0_.DESCRIPTION as DESCRIPT4_0_, document0_.DOC_STATUS_CODE as DOC5_0_, document0_.DOC_TYPE_CODE as DOC6_0_, document0_.DOC_DATE as DOC7_0_ from CY_DOCUMENT document0_ where document0_.DESCRIPTION='One-2-One test'
Hibernate: select estimate0_.ESTIMATE_ID as ESTIMATE1_0_, estimate0_.DOCUMENT_ID as DOCUMENT2_1_0_ from CY97212_ESTIMATE estimate0_ where estimate0_.DOCUMENT_ID=?
Hibernate: select estimate0_.ESTIMATE_ID as ESTIMATE1_0_, estimate0_.DOCUMENT_ID as DOCUMENT2_1_0_ from CY97212_ESTIMATE estimate0_ where estimate0_.DOCUMENT_ID=?
Hibernate: select estimate0_.ESTIMATE_ID as ESTIMATE1_0_, estimate0_.DOCUMENT_ID as DOCUMENT2_1_0_ from CY97212_ESTIMATE estimate0_ where estimate0_.DOCUMENT_ID=?
May 9, 2006 6:01:44 PM com.mitchell.hibernatetest.test.DocumentDAOTester testFindDocuments
INFO: doc id: 1473
May 9, 2006 6:01:44 PM com.mitchell.hibernatetest.test.DocumentDAOTester testFindDocuments
INFO: doc id: 1470
May 9, 2006 6:01:44 PM com.mitchell.hibernatetest.test.DocumentDAOTester testFindDocuments
INFO: doc id: 1476

So in order not to get n+1 selection, I guess I have to choose back to use one-to-many here. At least I get the lazy initialization befenit.

Any recommendation?

I believe I am having the same problem with 3.2 CR2. I can eliminate the N+1 queries if I change from bidirectional one-to-one to bidirectional one-to-many. Any better ideas?

Have you tried <one-to-one ...... outer-join="false" />

This disables fetching using an outer join (even if proxying is disabled says the documentation).