How do I get unique elements in this array

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

User · Answer

Instead of using an Array  consider using either a Hash or a Set   Sets behave similar to an Array  only they contain unique values only  and  under the covers  are built on Hashes  Sets don t retain the order that items are put into them unlike Arrays  Hashes don t retain the order either but can be accessed via a key so you don t have to traverse the hash to find a particular item   I favor using Hashes  In your application the user id could be the key and the value would be the entire object  That will automatically remove any duplicates from the hash   Or  only extract unique values from the database  like John Ballinger suggested

User · Answer

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

User · Answer

For those hitting this up in the future  you can now use the Mongoid  Criteria distinct method from Origin to select only distinct values from the database     Requires a Mongoid  Criteria Attendees all distinct  user id    http   mongoid org en mongoid docs querying html  v3 1 0

User · Answer

Have you looked at this page   http   www mongodb org display DOCS Aggregation Aggregation-Distinct  That might save you some time    eg db addresses distinct  zip-code

User · Answer

This should work for you   Consider Table1 has a column by the name of activity which may have the same value in more than one record  This is how you will extract ONLY the unique entries of activity field within Table1    An array of multiple data entries  table1   Table1 find  all     extracts   activity   for each entry in the array  table1  and returns only the ones which are unique    unique activities    table1 map  t  t activity  uniq

User · Answer

Errr  it s a bit messy in the view  But I think I ve gotten it to work with group  http   mongoid org docs querying    Controller   event attendees   Activity only  user id  where  action   gt   Attend   order by  created at desc  group   View   lt    event attendees each do  event attendee    gt         lt    event attendee  group   first user first name   gt   lt   end   gt

[ruby-on-rails] How do I get unique elements in this array?

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