class Parent < ApplicationRecord
  has_many :children
end

class Child < ApplicationRecord
  belongs_to :parent
  validates :name, uniqueness: true
end

parent = Parent.new(children: [Child.new(name: 'Wiske'), Child.new(name: 'Wiske')])
parent.save
parent.errors.any?

parent = Parent.new(children: [Child.new(name: 'Wiske'), Child.new(name: 'Wiske')])
parent.valid? # true
  # Child Exists (0.6ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
  # Child Exists (0.5ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
parent.save # true
  #  (0.6ms)  BEGIN
  # Child Exists (0.6ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
  # Child Exists (0.5ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
  # Parent Create (0.5ms)  INSERT INTO `parents` (`created_at`, `updated_at`) VALUES ('2018-05-20 19:14:06', '2018-05-20 19:14:06')
  # Child Exists (0.6ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
  # Child Create (7.2ms)  INSERT INTO `children` (`name`, `parent_id`) VALUES ('Wiske', 5)
  # Child Exists (0.6ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
  #  (2.8ms)  COMMIT
parent.errors.any? # false
parent.children # [#<Child id: 6, name: "Wiske", parent_id: 5>, #<Child id: nil, name: "Wiske", parent_id: 5>]
parent.children[1].errors.any? # true
parent.children[1].errors.full_messages # ["Name has already been taken"]
parent.valid? # false
  # Child Exists (0.7ms)  SELECT  1 AS one FROM `children` WHERE `children`.`name` = BINARY 'Wiske' LIMIT 1
parent.errors.any? # true
parent.errors.full_messages # ["Children is invalid"]