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please consider improving usage of find with fields parameter specified to handle DBRefs. This can be easy way to do joins automatically like this:

> db.foo.insert({a: 1});

> var a = db.foo.findOne();

> a

{ "_id" : ObjectId("4c67d3ae8d551a734559d73c"), "a" : 1 }

> db.foo.insert({b:2, a: new DBRef("foo", a._id)});

> var b = db.foo.findOne({b:2});

> b

	"_id" : ObjectId("4c67d3ee8d551a734559d73d"),

	"b" : 2,

	"a" : {

		"$ref" : "foo",

		"$id" : ObjectId("4c67d3ae8d551a734559d73c")

improved find would be able to do this:

> var b = db.foo.findOne({b:2}, {b:1, "a.a": 1});

> b

{ "_id" : ObjectId("4c67d3ee8d551a734559d73d"), "b" : 2, "a" : { "_id" : ObjectId("4c67d3ae8d551a734559d73c"), "a" : 1} }

duplicates

SERVER-19095 $lookup

is related to

SERVER-745 Embedded Document Expansion (Pseudo-JOINs)

SERVER-432 server side expansion of DBRefs