ActiveRecord::QueryMethods 中 joins 的一种用法
要根据一个 role，找到所有 user。
class User < ActiveRecord::Base
has_many :members
has_many :roles, through: :members
scope :by_role, ->(role) { joins(:roles).where( roles: { id: role }) }
end
class Member < ActiveRecord::Base
belongs_to :user
has_many :member_roles
has_many :roles, through: :member_roles
end
class MemberRole < ActiveRecord::Base
belongs_to :role
belongs_to :member
end
class Role < ActiveRecord::Base
end
查询可以这样
User.by_role(2).to_sql
生成的 SQL
"SELECT "users".* FROM "users" INNER JOIN "members" ON "members"."user_id" = "users"."id" INNER JOIN "member_roles" ON "member_roles"."member_id" = "members"."id" INNER JOIN "roles" ON "roles"."id" = "member_roles"."role_id" WHERE "roles"."id" = 2"
也可以改成
class User < ActiveRecord::Base
has_many :members
scope :by_role, ->(role) { joins(members: :roles).where( roles: { id: role }) }
end
