How would I select all embedded documents of the same type in mgo Mongodb?

I have a go application which uses mgo/mongodb. I'm using embedded documents rather than relational ones.

So I have... (some code redacted for brevity).

type User struct {
	Id          bson.ObjectId `bson:"_id,omitempty" json:"id"`
	Name        string        `form:"name" bson:"name" json:"name"`
	Password    string        `form:"password" bson:"password,omitempty" json:"password" binding:"required"`
	Email       string        `form:"email" bson:"email,omitempty" json:"email" binding:"required"`
	Artists     []Artist      `form:"artists" bson:"artists,omitempty" json:"artists" inline`
	Releases    []Release     `form:"releases" bson:"releases,omitempty" json:"releases" inline`
	ContentFeed []Content     `form:"content_feed" bson:"content_feed,omitempty" json:"content_feed" inline`
	Profile     Profile       `form:"profile" bson:"profile,omitempty" json:"profile" inline`
	TopTracks   []Track       `form:"top_tracks" bson:"top_tracks" json:"top_tracks" inline`
}

type Artist struct {
	Id     bson.ObjectId `bson:"_id,omitempty" json:"id"`
	Title  string        `form:"title" bson:"title" json:"title"`
	Genres string        `form:"genres" bson:"genres" json:"genres"`
}

func (repo *ArtistRepo) GetArtists() ([]Artist, error) {
    results := &[]Artist{}
	err := repo.collection.Find(???).All(results)
	return results, err
}

I'm trying to get all of the artists, from all of the users essentially. But I can't figure what I need in my query? I've touched briefly on Map/Reduce, but it didn't seem to apply to what I'm trying to do.

I think you are assuming mgo is an "ORM". But it's just a simple way to store data in Mongo. There are 3 different ways to fix your problem:

1. Put different types into different collections. That way, every document is the same type. (Collections are like "tables" in a relational database).

2. Tag each thing with it's type (i.e. store the object type in a field), then you can query on it.

3. If you are feeling dangerous, you can assume all Artists have Genres, and all Users have a Profile. Then use $exists to select only that type.

The first option is the usual way to do it. You should have specific reasons for doing #2 or #3, as they could be slower.