Re: Using distinct in to-many relationship?
Re: Using distinct in to-many relationship?
- Subject: Re: Using distinct in to-many relationship?
- From: Chuck Hill <email@hidden>
- Date: Tue, 22 Aug 2006 13:14:04 -0700
On Aug 22, 2006, at 7:04 AM, Marcos Trejo Munguia wrote:
Actually I,m using the EOFetchSpecification constructor that
pass that parameter:
public EOFetchSpecification(String entityName,
EOQualifier qualifier,
NSArray sortOrderings,
boolean usesDistinct,
boolean isDeep,
NSDictionary hints)
I,m passing true in usesDistinct, I suppose that it have the
same effect as setUsesDistinct(true). The problem is not with
the main entity but with the relationships passed in
setPrefetchingRelationshipKeyPaths, more specifically with to-
many relationships.\
I am wondering if you are mis-understanding something. The
distinct setting only applies to the objects that the fetch spec
returns. The objects returned don't include the pre-fetched
objects. Those are just fetched and cached, accessible by
navigating relationships. The objects that the fetch returns
are only those of entityName specified by qualifier.
I've discovered that today, also I've discovered that pre-fetched
objects in to-one relationships are fetched using DISTINCT and to-
many relationships don't. I know this because I'm debugging the
generated SQL. My model looks like this:
A <<-> B <->> C
I do the fetch of A, and pre-fetch of B and C, I'm getting
duplicates of C.
You may be fetching duplicates of C but EOF should not be creating
duplicate objects. The fetching of duplicate rows is possibly a
performance issue but should not affect the EO space.
Actually duplicate EOs are being created,
I doubt that they are. :-) Lists may have the same object in them
more than once, but that does not mean that EOF created duplicate
objects. If you find two objects that look to be the same, but which
have different EOGlobalID values, then EOF has created a duplicate.
Otherwise, what you have is the same object in a relationship
multiple times and that is a very different problem.
I noticed that because I'm using the to-many relationship in a
WORepetition.
It still sounds to me like you have a modeling or code error in those
relationships. I have never seen EOF include an object multiple
times unless I have somehow confused it.
Just one more question, if I do the fetch of objects of C in
another fetch specification using DISTINCT, the fetched objects are
available through A->B->C or another fetch is performed?
If you just navigate relationships, EOF will used the cached data --
unless the cached data has expired by the time you navigate the
relaltionship.
Again, look at batch faulting.
Chuck
How are you determining that you are getting duplicate objects
in the prefetched to-many relationships? I suspect that your
bug lies elsewhere.
I ran the generated SQL directly in my db console, there is where
I saw that I was getting duplicates.
It may be worth considering batch faulting as an alternative to
prefetching.
Chuck
On Aug 21, 2006, at 1:41 PM, Zak Burke wrote:
Marcos Trejo Munguia wrote on 8/21/06 2:17 PM:
Recently I found that the SQL generated by a fetch using
setPrefetchingRelationshipKeyPaths with a to-many
relationship generates
the query without distinct while for to-one relationships
uses distinct.
Call setUsesDistinct(true) on your fetch-spec.
I just discovered this too. It was the source of some very
mysterious
seeming behaviour -- extra copies of EOs would show up in
lists with
(seemingly) no explanation.
From the javadoc:
public void setUsesDistinct(boolean usesDistinct)
Sets whether duplicate objects or records are removed after
fetching. If flag is true they are removed (that is, proper sets
are returned). If flag is false they aren't (multisets are
returned). EOFetchSpecifications by default don't use distinct.
zak.
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