I have a has_many through relationship between my cities and travels that looks like this:

class Travel < ActiveRecord::Base
  has_many :cities, through: :destinations
class City < ActiveRecord::Base
  has_many :travels, through: :destinations

And on my Travel controller I have a method to join all its cities names:

  def city_names
    self.cities.map { |e| e.name }.join(', ')

I installed the bullet gem and it tells me that this is a n+1 query and suggests that I add the includes method on the query, but as far as I can tell this method will only work if I call it directly on the Travel class.

  • \$\begingroup\$ Don't use the meaningless variable e; use something like city instead \$\endgroup\$ Mar 19, 2015 at 1:04
  • \$\begingroup\$ Did you try calling includes on self in the city_names method? \$\endgroup\$ Mar 19, 2015 at 1:41
  • \$\begingroup\$ even better, just call map with (&:name) instead of a block \$\endgroup\$ May 29, 2015 at 13:05

1 Answer 1


You can use includes on any activerecord relation.

Judging by your message about having N+1 queries I suspect you are calling city_names somewhere in a loop. then the correct placement of the includes is the place where you select the elements to loop past.

I'm guessing it is for a view or something so i write some example code for .html.erb.

In controller:

@travels = Travels.all.includes(destinations: :cities)

In view:

<% @travels.each do |travel| %>
  ... Some code displaying travel info
  <%= travel.city_names %>
  ... More code displaying travel info
<% end %>

And a suggestion in line with what Mark Thomas wrote as a comment. You can refactor you city_names method for more readable code, either according to his suggestion or like this:

def city_names
  self.cities.map(&:name).join(', ')

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