db: preload embedded fields

[xiam]

The idea behind preloading is being able to map results from a join into an embedded type with ease:

artistCollection:=sess.Collection("artist")
publicationCollection:=sess.Collection("publication")
typeartistAssocstruct {
IDuint`db:"id"`...
}
typepublicationAssocstruct {
IDuint`db:"id"`...Author*artistAssoc`db:"author,omitempty,assoc"`
}
varpublications []publicationAssoc// Joining two db.Result objects by using Assoc.q:=publicationCollection.Find().Assoc(
artistCollection.Find("publication.author_id = artist.id"),
"author",
)

My proposal is adding an Assoc method which takes another db.Result from the same session and creates a JOIN behind the scenes. The second argument for Assoc is the name of the embedded field, in this case publicationAssoc has a Author *artistAssoc db:"author,omitempty,assoc"' field, that's why we use author as second argument for Assoc().

[VojtechVitek]

I'm not fully convinced this should be part of upper/db.

Same thing can be achieved with .Join().On() - am I right?

[xiam]

@VojtechVitek Yes, except there is no join for db.Result so it won't work with Find(). Peter has expressed similar feelings towards another magical feature #394 (comment), and I have to admit that moving this to bond (which is more opinionated) sounds like a good idea.