Question

Using Mongoid. Unfortunately, Mongoid does not allow for selecting unique / distinct!
Have gotten these results. As you can see, there are 7 results. If you look carefully (at user_id), there are only 2 users.

[
  #<Activity _id: 4cea6c4572357e00fa00011a, created_at: 2010-11-22 13:12:37 UTC, updated_at: 2010-11-22 13:12:37 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea447472357e00fa00009a')>, 
  #<Activity _id: 4cea6c3072357e00fa000116, created_at: 2010-11-22 13:12:16 UTC, updated_at: 2010-11-22 13:12:16 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea447472357e00fa00009a')>, 
  #<Activity _id: 4cea6bdd72357e00fa00010d, created_at: 2010-11-22 13:10:53 UTC, updated_at: 2010-11-22 13:10:53 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea447472357e00fa00009a')>, 
  #<Activity _id: 4cea46df72357e00fa0000a4, created_at: 2010-11-22 10:33:03 UTC, updated_at: 2010-11-22 10:33:03 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea447472357e00fa00009a')>, 
  #<Activity _id: 4cea40c572357e00fa00006f, created_at: 2010-11-22 10:07:01 UTC, updated_at: 2010-11-22 10:07:01 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea3c8b72357e00fa00005e')>, 
  #<Activity _id: 4cea3ca172357e00fa000062, created_at: 2010-11-22 09:49:21 UTC, updated_at: 2010-11-22 09:49:21 UTC, action: "Attend", user_id: BSON::ObjectId('4cea39b772357e00fa000046'), artist_id: nil, media_id: BSON::ObjectId('4cea3c8b72357e00fa00005e')>, 
  #<Activity _id: 4cea344a72357e00fa00003f, created_at: 2010-11-22 09:13:46 UTC, updated_at: 2010-11-22 09:13:46 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea306c72357e00fa000031')>
] 

I was looking at this, and was thinking I could do something similar so that my array would now look like this:

[
  #<Activity _id: 4cea6c4572357e00fa00011a, created_at: 2010-11-22 13:12:37 UTC, updated_at: 2010-11-22 13:12:37 UTC, action: "Attend", user_id: BSON::ObjectId('4cea2fb872357e00fa000025'), artist_id: nil, media_id: BSON::ObjectId('4cea447472357e00fa00009a')>, 
  #<Activity _id: 4cea3ca172357e00fa000062, created_at: 2010-11-22 09:49:21 UTC, updated_at: 2010-11-22 09:49:21 UTC, action: "Attend", user_id: BSON::ObjectId('4cea39b772357e00fa000046'), artist_id: nil, media_id: BSON::ObjectId('4cea3c8b72357e00fa00005e')>
]

I’m not concerned which combination of results are extracted. As long as I have unique user_id’s in the result set. Anyone know how this can be achieved?

Answer 1

You can just use the method uniq. Assuming your array is ary, call:

ary.uniq{|x| x.user_id}

and this will return a set with unique user_ids.