How to use group/group_by in Rails properly for Chartkick?

I am trying to display completed tasks done my user's friends. I have a function friend? that returns a boolean value

For Bar chart I need to use count

<%= bar_chart @completed_tasks.count%>

Here is how I try to get the desired data:

@completed_tasks = Task.where(completed: true).select{|task| friend?(task.user_id)}

And after all I need to group it by user_id. How can I do it properly?

I have tried adding the following options:

1. .group(:user_id)
2. .group_by(:user_id)
3. .group_by(&:user_id)

but i don't get any data represented in bar chart. What am I doing wrong?

The problem was in the fact that .group() works with ActiveRecord Relations. Task.where(...) is an ActiveRecord Relation, while select returns an array. Here is my solution:

arr = Task.where(completed: true).select{|task| User.find(task.user_id).is_friend?(current_user)}
      @completed_tasks =  Task.where(id: arr.map(&:id)).group(:user_id)