How to match mongoid documents using array fields in query?

Question

I am trying to match documents with Mongoid/Mongodb where array fields are used in the query. I've been struggling with $elemMatch but can't seem to get it.

Context

A Project can have admin, member, reader users
These users are referenced by a Project (HABTM)
I want to be able to find projects where:
- User A is admin
- User B is admin or member
- .. etc ..

Example

Given a Project document from Rails console:

[#<Project _id: 4f44355a9f5b7f385a000003, 
  _type: nil, name: "Project ABC", 
  desc: "some description", 
  admin_ids: 
    [BSON::ObjectId('123')], 
  member_ids: 
    [BSON::ObjectId('456'),
    BSON::ObjectId('789')], 
  reader_ids: []
>]

I had the following code:

@projects = Project.any_of({:admin_ids => [current_user.id]}, 
                           {:member_ids => [current_user.id]}).entries

Which matches current_user.id across either admin_ids and member_ids as long as there was only a single value in either of the arrays. As per the code above:

Trying to match user '123' gives correct result
Trying to match user '456' gives no result (incorrect)

$elemMatch

Based on researching, I think I should be using $elemMatch but am missing something.

As per the Project document code above:

// test case: this works with array of one
Project.all(conditions: {:admin_ids => "123"}).entries

// failure case: empty result   
Project.all(conditions: {:member_ids => {'$elemMatch' => {:id => '456' } }}).entries

// failure case: empty result
Project.all(conditions: {:member_ids => {'$elemMatch' => {:id => BSON::ObjectId('4f44a4019f5b7f3d5200000d') } }}).entries

I think you should just be able to use $in rather than $elemMatch - as the mongodocs say: "You only need to use this when more than one field must be matched in the array element." — Russell
– Russell, Commented Feb 22, 2012 at 11:16

rubish · Accepted Answer · 2012-02-22 13:50:49Z

11

You need to get rid of the array while querying.

@projects = Project.any_of({:admin_ids => current_user.id}, 
                       {:member_ids => current_user.id}).entries

That should work.

answered Feb 22, 2012 at 13:50

rubish

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1 Comment

tirdadc Over a year ago

for some reason I had to explicitly cast the current_user.id to a string with to_s for this to work in a scope.

Russell · Accepted Answer · 2012-02-22 11:21:28Z

11

I think you should just be able to use $in rather than $elemMatch - as the mongodocs say:

"You only need to use [$elemMatch] when more than one field must be matched in the array element."

Have you tried something like the following?

Project.any_in(:member_ids =>  [ member_id_one, member_id_two ])

answered Feb 22, 2012 at 11:21

Russell

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2 Comments

Daniel May Over a year ago

Thanks for the clarification around $elemMatch. Yeah, I needed to have something more like @rubish's code.

leetheguy Over a year ago

Alternatively you can do: Project.where(:member_ids.in => [ member_id_one, member_id_two ])

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