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

Answer 1

You can do as following:

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

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Be carefull 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' })
                #^^^           ^

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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

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

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

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