Store redundant fields in references

[alcaeus]

References in MongoDB can be just IDs for a referenced document, or they can be full-fledged DBRef objects that can contain additional information. We use this functionality to add a discriminator value to properly hydrate a referenced document.

This PR allows developers to store additional fields from a referenced document within the reference, so it can be used in queries. There are two caveats at this time:

• The denormalized value won't be updated. This would require us to keep track of incoming references to a document and update all documents referencing ours when such a field is updated. Apart from the potentially long update query, this would only work within the first nesting level of documents due to SERVER-831.
• The value is not yet used when creating proxy classes. This is because the current implementation of the proxy library doesn't allow for partially initialized proxies. This has been discussed in Lazy-load on a per-property base (removes doctrine proxies, replaced by ProxyManager) orm#1241 and I'd like to add that to ODM 2.0.

[malarzm]

Should we limit what can be stored in redundant field? I guess putting there EmbedMany field will have some side effects depending on it being initialzed or not

[malarzm]

Looks cool, putting into 1.2 feature bucket 👍

[malarzm]

Leaving a note to future us:

• need to check if storing association (can't recall if getField from class metadata can give us association) is safe
• allowing dot notation in redundant fields sounds interesting (not sure how helpful this could be but https://github.com/doctrine/mongodb-odm/pull/970/files introduced ReferencePrimer::parseDotSyntaxForPrimer which may or may not be useful :) )

[alcaeus]

Dot notation would be a bit complicated, since that may trigger a whole bunch of database loads. There's already a potential database load involved when building a DBRef object (e.g. when the referenced document is not initialized and redundant fields are defined). This happens even when the DBRef object is not used for writing, but only for reading (e.g. by being called in Query\Builder::references. Allowing dot notation and thus a whole reference tree would only make the problem worse. This is something I'd like to address, but I'm not sure it can be done in a BC fashion.

[alcaeus]

Closing as the PR is horribly stale: created #2252 to track this and will try to revisit this for one of the next 2.x releases.