[DDC-16] DQL Ignores properties of subclasses Created: 15/Sep/09  Updated: 22/Jul/14  Resolved: 24/Feb/13

Status: Closed
Project: Doctrine 2 - ORM
Component/s: DQL
Affects Version/s: 2.0-ALPHA2
Fix Version/s: None
Security Level: All

Type: Improvement Priority: Minor
Reporter: Avi Block Assignee: Marco Pivetta
Resolution: Won't Fix Votes: 6
Labels: None

Issue Links:
Duplicate
is duplicated by DDC-1377 Doctrine doesn't understand associati... Closed

 Description   

I have a classes B and C which inherit from superclass A. I would like
to get a list of all A's but filter the list to ignore those in C
which have a property d set to 2.

select a from A where a.d == 2 fails because "d" is not a property of A.



 Comments   
Comment by Roman S. Borschel [ 31/Mar/10 ]

Thats indeed tricky. That syntax alone can, however, never work, because there might be several subclasses that have a field named "d", so Doctrine would not know which field you mean.

We might need special syntax for such constructs but I'm not sure it is worth it. If anyone has an idea, just shoot.

Alternatively, apart from using a native query, what about this DQL:

select a from A a where a.id not in (select c.id from C c where c.d = 2)
Comment by Benjamin Eberlei [ 01/Apr/10 ]

I am sorry, but isn't this sort of query a violation of object oriented programming, Accessing a Superclass A and checking for a property that exists on a certain child is not possible in any strict OO language without specific casting into the type beforehand.

Comment by Roman S. Borschel [ 01/Apr/10 ]

Thats why I said "we would need special syntax for such constructs", i.e. casting syntax

However, the alternative query I showed should work just fine for such purposes.

Comment by Menno Holtkamp [ 18/Jul/11 ]

+1 for this request.

When displaying data (stored in a database) to end-users using generated tables, server-side sorting and filtering on properties that belong to subclasses is a common task, for instance to provide the data presented in a JQuery Datatable: http://www.datatables.net.

For this purpose, DQL is great to generate the required queries dynamically. However, since properties of subclasses can currently not be 'accessed', sorting and filtering on these properties becomes impossible, or at least cumbersome to implement. Currently I have lifted/copied specific properties to a general 'value' property of the superclass, which CAN be used for ordering and filtering. Of course, this work-around significantly decreases the power of inheritance strategy in the originally envisioned model.

In Hibernate this seems to have been solved in HQL using downcasting, which seems a viable approach, but as Benjamin already mentioned not a best practice in a strict OO approach. Therefore a specific syntax might be required to hint on the to-be-expected casts:

For additional motivation of this subject, check a recent post of mine at the user groups: http://groups.google.com/group/doctrine-user/msg/caebf8139e06e01a

Comment by Avi Block [ 18/Jul/11 ]

Honestly I gave up on all of this a long time ago, and now use a CQRS approach. I make views, or denormalized tables for every view in my application, and just use straight SQL.

When I need to do creating or updating or whatever, I use doctrine.

Comment by Glen Ainscow [ 19/Jul/11 ]

I would like this as well (though my use case involves joining to the subclass).

Registration has one Opponent (superclass to Player and TeamSelection). I would like to count the number of registrations for a particular team within a specific tournament. For example:

SELECT COUNT(*)
FROM Registration r
INNER JOIN r.opponent CAST TeamSelection ts
INNER JOIN ts.team t
WHERE r.tournament = 1
AND t.name = "Team #1"
Comment by Glen Ainscow [ 19/Jul/11 ]

Thinking about this a bit more, it can actually improve query performance, as it would no longer be necessary to join the other subclasses. In other words, since you're casting the opponent to a TeamSelection, it's not necessary to join to the players table.

A cast should probably also restrict the result using the discriminator column, for example (SQL):

INNER JOIN opponents o ON o.id = r.opponent_d AND o.type = "TeamSelection"
Comment by Guilherme Blanco [ 15/Sep/11 ]

Possible solution:

SELECT p FROM CAST(Person AS User) p

Or:

SELECT a, p FROM Article a JOIN CAST(a.person AS User) p
Comment by Glen Ainscow [ 16/Sep/11 ]

If you're casting a Person (superclass) to a User (subclass), then shouldn't the alias be "u" and not "p"?

i.e. SELECT a, u FROM Article a JOIN CAST(a.person AS User) u

Comment by Benjamin Eberlei [ 16/Sep/11 ]

the alias is that, an alias, you can use whatever you want.

Comment by Glen Ainscow [ 16/Sep/11 ]

Yes I know that, I'm just making sure that I understand the syntax.

Comment by Jaka Jancar [ 18/Feb/12 ]

I just ran into a similar problem and am not sure the above cast solution would do.

I'm trying to do:

SELECT
    s
FROM
    Superclass s
WHERE
    s INSTANCE OF SubclassA OR
    s INSTANCE OF SubclassB AND s.foo = 'bar';

So I'd need to use CAST in a WHERE condition, not in the FROM/JOIN clause, e.g:

SELECT
    s
FROM
    Superclass s
WHERE
    s INSTANCE OF SubclassA OR
    s INSTANCE OF SubclassB AND (CAST s AS SubclassB).foo = 'bar';
Comment by Wladimir Coka [ 03/Apr/12 ]

Is there any workaround? Would be perfect if I could just:

 SELECT a, p FROM Article a JOIN CAST(a.person AS User) p 

(+1 for Guilherme Blanco possible solution)

Comment by Wladimir Coka [ 04/Apr/12 ]

I was trying a workaround:

Before executing the dql query, I change the mapping info of the association to the concrete type that I was going to use. Like this:

        $cmf = $this->em->getMetadataFactory();
        $class = $cmf->getMetadataFor("Article");
        $class->associationMappings["person"]["targetEntity"]="User";

But now my problem is that I'm actually using an inverse side (in query where this is needed), so when I change the targetEntity, the sql join that is generated, is using the table of the concrete class (User in this case) and not the base class (Person)

class Person {
...
	/**
	 * @OneToOne(targetEntity="Order",cascade={"persist"})
	 * @JoinColumn(name="order_id", referencedColumnName="orden_id")
	 */
	private $order;
...
}

class Order {
...
    /**
     * @OneToOne(targetEntity="Person", mappedBy="order")
     */
    private $person;
...
}

The SQL generated is joining as if the "user" table has orden_id field

Comment by Marco Pivetta [ 24/Feb/13 ]

I am closing this one.

The requirement of this issue is basically violating OO principles.

If you really need to filter across multiple child-entities in your inheritance, then try something as following instead:

SELECT
    r
FROM
    Root r
WHERE
    r.id IN (
        SELECT
            c.id
        FROM
            Child c
        WHERE
            c.field = :value
    )

Up-casting is not really a viable solution anyway, since it would be even more weird to cast in DQL and then have hydration still retrieve the root entity type.

Comment by Bogdan Yurov [ 17/Jul/14 ]

@Marco You are wrong. There is no way to do this. It is just not wise to deny a feature just because it violates OO principles when this is the only method.

There are usecases, when we have nothing to do. For example, we are searching through the whole table in STI. We use quick filters like "Like" so there is not need in difficult indexes/additional tables for them. We CAN NOT in ANY way overcome this limitation when we need to use fields of different classes.

This is certainly not a bug, when we may say "WONT FIX" and just ignore all those people who face it.

Comment by Marco Pivetta [ 17/Jul/14 ]

It is just not wise to deny a feature just because it violates OO principles when this is the only method.

We are working on an ORM (Object-Relational-Mapper), therefore Object-Orientation is first-class citizen and prioritized over everything else in the ORM.

There are usecases, when we have nothing to do.

There are use-cases for everything, but that doesn't mean that they should clutter the tool. This does not only apply to Doctrine ORM. An edge case functionality that has multiple workarounds and that cannot be implemented correctly within the tool does NOT need to land in the tool.

I provided a workaround that respects the OO and ORM rules and works also with the OO paradigm. Here is a pseudo-code description of what the DQL I pasted above does:

  • select all objects that are instance of Child by $someCriteria
  • select all objects that are instance of Root
  • iterate over Root instances and filter out those that are not in the results of the first selection

We CAN NOT in ANY way overcome this limitation when we need to use fields of different classes.

Yes you can: there's SQL. SQL is a "structured query language" that is weakly typed and allows this sort of operation natively. This is a problem of the SQL domain, and it should be solved with SQL whenever the DQL becomes too complex/limited. Don't try to stuff a feature that is only possible in the SQL domain into DQL just because they look similar.
They are not the same thing.

This is certainly not a bug, when we may say "WONT FIX" and just ignore all those people who face it.

We are not ignoring the problem: we just point you to the right scope/tools to solve it. DQL has one way of solving it, which may or may not be viable to you. SQL has another way of solving this same problem, and it's not going to be implemented in DQL because DQL is statically typed, and upcasts and type juggling are not going to land in DQL.

Comment by Bogdan Yurov [ 17/Jul/14 ]

If we provide a workaround, is there any way to merge it to master branch? What we can do to vote up this feature?

Comment by Marco Pivetta [ 17/Jul/14 ]

Bogdan Yurov we already tried looking into this multiple times.

The only "clean-ish" solution would be to support upcasting somehow, and the effort and complexity derived from it is not worth it.

Comment by Bogdan Yurov [ 17/Jul/14 ]

@ocramius
What about "CAST()" construction? You were talking about OO, but there are some languages, that could argue with you.

For example, Delphi. It supports casting manually to selected type (for example TObject -> TButton). Does it break best violates OO principles too? No, of course.
Another example is Java. It has exactly same language construction for exactly same purpose. Is doctrine more concentrated on OO principles than Java or Delphi?

Comment by Marco Pivetta [ 22/Jul/14 ]

Bogdan Yurov I think Guilherme Blanco is thinking of providing a cast syntax.

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