Rails: order using a has_many/belongs_to relationship

Question

I was wondering if it was possible to use the find method to order the results based on a class's has_many relationship with another class. e.g.

# has the columns id, name
class Dog < ActiveRecord::Base
  has_many :dog_tags
end
# has the columns id, color, dog_id
class DogTags < ActiveRecord::Base
  belongs_to :dog
end

and I would like to do something like this:

@result = DogTag.find(:all, :order => dog.name)

thank you.

Solution 1

You need to join the related table to the request.

@result = DogTag.find(:all, :joins => :dog, :order => 'dogs.name')

Note that dogs is plural in the :order statement.

Solution 2

In Rails 4 it should be done this way:

@result = DogTag.joins(:dog).order('dogs.name')

or with scope:

class DogTags < ActiveRecord::Base
  belongs_to :dog
  scope :ordered_by_dog_name, -> { joins(:dog).order('dogs.name') }
end
@result = DogTags.ordered_by_dog_name

The second is easier to mock in tests as controller doesn't have to know about model details.

Comments

[Staelen]

should be DogTag.find(:all, :joins => :dog, :order => 'dogs.name') =)

[Evan]

Thank you for the solution. I'm sure you know, but for anyone else, I found that the order had to be the table name as well, i.e. I had to pluarlize it: 'dogs.name' not 'dog.name'

[Junchao Gu]

I tried the first but it did not work. Instead this works: @result = DogTag.joins(:dog).order('name'). I am using PostgreSQL, not sure if it is related

[Simon Franzen]

DogTab.where(...).joins(:dog).order('dogs.name') yeah ! Thanks !