Retrieving array of ids in Mongoid

Question

how do you retrieve an array of IDs in Mongoid?

arr=["id1","id2"]
User.where(:id=>arr)

You can do this easily if you are retrieving another attribute

User.where(:nickname.in=>["kk","ll"])

But I am wondering how to do this in mongoid -> this should be a very simple and common operation

score 50 · Accepted Answer · edited Dec 07 '18 at 13:20

50

Remember that the ID is stored as :_id and not :id . There is an id helper method, but when you do queries, you should use :_id:

User.where(:_id.in => arr)

Often I find it useful to get a list of ids to do complex queries, so I do something like:

user_ids = User.only(:_id).where(:foo => :bar).distinct(:_id)
Post.where(:user_id.in => user_ids)

edited Dec 07 '18 at 13:20

Alupotha

answered Dec 08 '10 at 18:10

bowsersenior

The solution above works fine when amount of users is small. But it will require a lot of memory while there are thousands of users. – dpaluy Oct 15 '12 at 20:30
@dpaluy you are right--this approach is only a hack. Also, the data can change between the first and second queries, especially when there are a lot of documents. Use with caution, or better yet, figure out a better way! – bowsersenior Oct 16 '12 at 19:28
4

The better way is to use `.distinct(:_id)` instead of `.map(&:_id)`. `distinct` works during executing the query which is slightly faster than telling rails to loop the array – Zakwan May 23 '14 at 17:06

score 5 · Answer 2 · answered May 18 '11 at 23:32

5

Or simply:

arr = ['id1', 'id2', 'id3']
User.find(arr)

answered May 18 '11 at 23:32

gjb

1

Issue with above: If just one id is not found, it will return error and not the ids that were found. – dman Jan 16 '15 at 18:24
`Mongoid.raise_not_found_error = false` will avoid document not found error raised on above query – r3bo0t Sep 23 '16 at 09:28

score 4 · Answer 3 · answered Jan 21 '11 at 09:47

4

The above method suggested by browsersenior doesn't seem to work anymore, at least for me. What I do is:

User.criteria.id(arr)

answered Jan 21 '11 at 09:47

Bryan L.

3

Just a quick note. With Mongoid 2.0.0 and later, you will need to use `for_ids` instead of `id` like so: `User.criteria.for_ids(arr)` – bowsersenior Apr 12 '11 at 04:13

score 2 · Answer 4 · answered Oct 15 '12 at 20:42

user_ids = User.only(:_id).where(:foo => :bar).map(&:_id)
Post.where(:user_id.in => user_ids)

The solution above works fine when amount of users is small. But it will require a lot of memory while there are thousands of users.

User.only(:_id).where(:foo => :bar).map(&:_id)

will create a list of User objects with nil in each field except id.

The solution (for mongoid 2.5):

User.collection.master.where(:foo => :bar).to_a.map {|o| o['_id']}

4 Answers4