mongodb: best way to get specific documents and then the rest - ruby-on-rails

Question

lets say I have 1000 documents where each one has:
user_id
text
Now, I would like to pull all those documents but first pull the documents from a few specific users (given an array of user ids) and then all the rest.
I was thinking to use map reduce to create a new weight inline attribute if the user_id exists in the specific users array (using scope to pass the array) and then to sort that new attribute. But from what I could understand, you can not sort after map reduce.
Any one has a good suggestion how to pull this off? Any suggestion will be welcome.
Thanks!

Answer 1

Well there isn't a lot of detail here, but I can give a sample case for consideration. Consider the following set of documents:
{ "user" : "fred", "color" : "black" }
{ "user" : "bill", "color" : "blue" }
{ "user" : "ted", "color" : "red" }
{ "user" : "ted", "color" : "black" }
{ "user" : "fred", "color" : "blue" }
{ "user" : "bill", "color" : "red" }
{ "user" : "bill", "color" : "orange" }
{ "user" : "fred", "color" : "orange" }
{ "user" : "ted", "color" : "orange" }
{ "user" : "ally", "color" : "orange" }
{ "user" : "alice", "color" : "orange" }
{ "user" : "alice", "color" : "red" }
{ "user" : "bill", "color" : "purple" }
So suppose you want to bubble the items for the users "bill" and "ted" to the top of your results, then everything else sorted by the user and the color. What you can do is run the documents through a $project stage in aggregate, as follows:
db.bubble.aggregate([
// Project selects the fields to show, and we add a weight value
{$project: {
_id: 0,
"user": 1,
"color": 1,
"weight": {$cond:[
{$or: [
{$eq: ["$user","bill"]},
{$eq: ["$user","ted"]}
]},
1,
0
]}
}},
// Then sort the results with the `weight` first, then `user` and `color`
{$sort: { weight: -1, user: 1, color: 1 }}
])
So what that does is conditionally assign a value to weight based on whether the user was matched to one of the required values. Documents that do not match are simply given a 0 value.
When we move this modified document on to the $sort phase, the new weight key can be used to order the results so the "weighted" documents are on top, and anything else will then follow.
There a quite a few things you can do to $project a weight in this way. See the operator reference for more information:
http://docs.mongodb.org/manual/reference/operator/aggregation/