How to complete a query in Rails with a deep condition? - sql

Question

I am attempting to query a database based on a deep condition. As an example, I would like to find all of the people whose primary pet has a specific breed name:
Person > Pet > Breed > Name
The models might look like this.
class Person
has_many pets
attr_accessor :primary_pet
end
class Pet
belongs_to Person
has_one Breed
end
class Breed
# no belonging relationship
attr_accessor :name
end
Now I could do:
Person.find_each do |person|
puts person if primary_pet.breed.name == 'Norwich Terrier'
end
but I want to figure out the proper query syntax for doing this with a where method. The following does not work:
Person.where(primary_pet.breed.name: 'Norwich Terrier').find_each do |person|
puts person
end

Answer 1

I formalized the model relationships with minor edit, and ended up with this:
class Person < ActiveRecord::Base
has_many :pets
attr_accessor :primary_pet
end
class Pet < ActiveRecord::Base
belongs_to :person
belongs_to :breed
end
class Breed < ActiveRecord::Base
# no belonging relationship
attr_accessor :name
end
The best query that I came up with (that actually worked) was this:
Person.joins(pets: :breed).where(breeds: { name: 'Norwich Terrier' }).where("people.primary_pet = pets.id")
This says to join the relationship for pets and to breeds. This allows using the breeds structure to filter for 'Norwich Terrier'. And to finish it off, to additionally filter by primary_pet_id related to pets.id.
That should get you what you asked for in the question.
Here's how I tested it:
breed_names = ["Beagle", "Collie", "Norwich Terrier", "German Shepard", "Dachshund"]
people_names = ["Barbara", "Connie", "Paula", "Gerald", "Dave"]
# Add breeds
breed_names.each {|name| Breed.create!(name: name) }
# Add people
people_names.each {|name| Person.create!(name: name) }
# Add some pets
people_names.each do |name|
person = Person.find_by(name: name)
breed_names.sample(3).each do |breed|
breed = Breed.find_by(name: breed_name)
Pet.create!(breed_id: breed.id, person_id: person.id)
end
end
# Assign the primary_pet for each person
Person.all.each {|person| pet = person.pets.first ; person.update!(primary_pet: pet) if pet }
# List all people and their pets by breed
Person.all.each do |person|
puts "#{person.name}: #{person.pets.map {|pet| pet.breed.name}}"
end
# List all people and their primary pet breed
Person.all.each do |person|
puts "#{person.name}: #{person.primary_pet ? person.primary_pet.breed.name : ''}"
end
# Run the query
Person.joins(pets: :breed).where(breeds: { name: 'Norwich Terrier' }).where("people.primary_pet = pets.id")