Add counter cache column for pull requests.
user
```ruby
class AddPullRequestCounterCacheColumnToUsers < ActiveRecord::Migration
  def change
    add_column :users, :pull_requests_count, :integer, default: 0

    User.reset_column_information
    User.all.each do |p|
      User.update_counters p.id, :pull_requests_count => p.pull_requests.count
    end
  end
end
```
assistant