How do I get unique elements in this array?

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.

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 

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)

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");

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.