test.html.erb

<%= @user_assignments.each do |assignment| %>

assignment.steps.size -- will do query for each assignment
It would be nice to do something like 
@user_assignments.preload_counts(:steps) and then just do assignment.steps_count

<% end %>

test.rb

class Assignment < ApplicationRecord
  belongs_to :course
  belongs_to :user
  has_many :steps, through: :course
end

class Course < ApplicationRecord
  belongs_to :course
  has_many :steps
end

class Step < ApplicationRecord
  belongs_to :course
end