How to query a model based on attribute of another model which belongs to the first model?

Question

If I have a model Person, which has_many Vehicles and each Vehicle can be of type car or motorcycle, how can I query for all persons, who have cars and all persons, who have motorcycles?

I don't think these are correct:

Person.joins(:vehicles).where(vehicle_type: 'auto')
Person.joins(:vehicles).where(vehicle_type: 'motorcycle')

Answer 1

You can do as following:

Person.includes(:vehicles).where(vehicles: { vehicle_type: 'auto' })
Person.includes(:vehicles).where(vehicles: { vehicle_type: 'motorcycle' })

---

Be careful with .joins and .includes:

# consider these models
Post # table name is posts
  belongs_to :user
                #^^
User # table name is users
  has_many :posts
               #^

# the `includes/joins` methods use the relation name defined in the model:
User.includes(:posts).where(posts: { title: 'Bobby Table' })
                  #^            ^
# but the `where` uses the exact table name:
Post.includes(:user).where(users: { name: 'Bobby' })
                #^^^           ^

---

A tricky one:

Post
  belongs_to :author, class_name: 'User'
User # table named users
  has_many :posts

Post.includes(:author).where(users: { name: 'John' })
# because table is named users

---

Alternatively, the gem activerecord_where_assoc can achieve this (and much more):

Person.where_assoc_exists(:vehicles, vehicle_type: 'auto')

---

Similar questions:

• association named not found perhaps misspelled issue in rails association
• Rails active record querying association with 'exists'
• Rails 3, has_one / has_many with lambda condition
• Rails 4 scope to find parents with no children
• Join multiple tables with active records
• Rails: Finding all Users whose relationship has a specified attribute