Doctrine2: [Semantical Error] Cannot select entity

问题:

This is my query with query builder, and it works perfectly, bringing all the results of user table and the modules table, which has a many to many association:

public function getUser($id){
    $qb = $this->getEm()->createQueryBuilder()
    ->select('u', 'm')
    ->from('Adm\Entity\User', 'u')
    ->join('u.modules', 'm')
    ->where('u.id = ?1')
    ->setParameters(array(1 => $id));
    $result = $qb->getQuery()->getResult(\Doctrine\ORM\AbstractQuery::HYDRATE_ARRAY);
    return $result;
}

But when i try to select specific fields from user like this:

public function getUser($id){
    $qb = $this->getEm()->createQueryBuilder()
    ->select('u.id, u.name, u.email', 'm')
    ->from('Adm\Entity\User', 'u')
    ->join('u.modules', 'm')
    ->where('u.id = ?1')
    ->setParameters(array(1 => $id));
    $result = $qb->getQuery()->getResult(\Doctrine\ORM\AbstractQuery::HYDRATE_ARRAY);
    return $result;
}

Doctrine throws an error:

[Semantical Error] line 0, col -1 near 'SELECT u.id,': Error: Cannot select entity through identification variables without choosing at least one root entity alias.

I would like to know how to do that, to select specific fields from the entity and not all the fields.

回答1:

The problem with your second query is that you are trying to return the associated modules objects but not the User object. Doctrine doesn't like this, and doesn't operate this way. I know you tried to hydrate an array, but if you hadn't, this is what your first query would be trying to return:

User object {
    ...,
    $modules -> array of Module objects
}

So you'd return a single User object, and then your $modules member of that User class is going to be an array of all associated module objects. Even though you are selecting u, m, Doctrine still wants to return that one object because m is just an association on u. Just because you want to hydrate an array doesn't change how Doctrine wants to select your data to begin with.

Your second example - Doctrine doesn't know how to return that. By individually selecting User fields, you are no longer returning a User object but an array of User values. So, where could it even return associated modules there? It wouldn't make sense to return this format:

[
    user id,
    user name,
    user email,
    [ array of Module objects ]
]

This is even trickier as a Many-to-Many association. The real question is, what are you trying to accomplish with your second query that you find unacceptable with your first query? If it's performance, you're probably not gaining much with the second query. If it's simply returning specific columns, then you should be specifying those m. columns as well.