问题
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')
回答1:
You can do as following:
Person.includes(:vehicles).where(vehicles: { type: 'auto' })
Person.includes(:vehicles).where(vehicles: { type: 'motorcycle' })
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' })
#^^^ ^
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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来源：https://stackoverflow.com/questions/23633301/how-to-query-a-model-based-on-attribute-of-another-model-which-belongs-to-the-fi