Fix query preparation when querying for referenced object

[alcaeus]

This PR fixes an issue introduced with #1363 when running queries like $dm->getRepository(User::class)->findOneBy(['account' => $account]);, with $account being a referenced document. When using the references method of the query builder, the query array looks like this:

[
    'account.$id' => MongoId(...)
]

When using includesReferenceTo which is made for @ReferenceMany mappings, the result looks like this:

[
    'account' => [
        '$elemMatch' => [
            '$id' => MongoId(...)
        ]
    ]
]

However, the implementation in the DocumentPersister always yielded a full DBRef object:

[
    'account' => [
        '$ref' => 'accounts',
        '$id' => MongoId(...)
        '$db' => 'mydatabasename'
    ]
]

This caused issues when forcing queries to be covered by indexes (through the notablescan option in MongoDB), since it would require two different kinds of indexes on a reference: one on the entire DBRef object to cover queries like created through the DocumentPersister and another index on the $id field of the reference to cover queries created by the query builder in combination with references.

This solution tries to apply the same logic applied by the query builder to the documents in question. The only exception are @ReferenceOne fields without a target document set. These would produce the following query array:

[
    'account.$ref' => 'accounts',
    'account.$id' => MongoId(...),
    'account.$db' => 'mydatabasename'
]

This is not possible at this time since the DocumentPersister always assumes that a single field has exactly one value. For now, the PR includes a failing test case for this, as the DocumentPersister excludes @ReferenceOne fields without targetDocument set.

[malarzm]

I had to stop for a moment and think about why such a check is here, the original check on strategy is way more readable. Also it will prevent us from hitting an edge case when id might be an array

[malarzm]

As a starter I'm sorry for introducing the bug, I admit that I haven't thought about indexes at all back then :)

This is not possible at this time since the DocumentPersister always assumes that a single field has exactly one value.

If this is only issue of what's prepareQueryElement is returning (and how) it could be fairly easy (found 3 places using the method) to rework it so it could return an array of tuples (or ["field.$id" => abc, "field.$ref" => xyz] array) - what do you think?

[malarzm]

This should be ===

[alcaeus]

it could be fairly easy to rework it so it could return an array of tuples

I thought about that as well - just wasn't sure which route we want to take. I'll have to see how we can accomodate that without creating a weird return type, but if at all possible I'll do exactly that. Thanks for the input! 👍

[alcaeus]

@malarzm udpated PR as per your suggestion. prepareQueryElement now returns an array of key-value tupels so that more than one query element can be returned.

[malarzm]

Nitpicking, but you have a typo in var name ;)

[alcaeus]

It's not a typo, it's German! 😁

[malarzm]

So the germanization of Doctrine begins, one variable at a time? :P

[malarzm]

Thanks \o/