gistfile1.rb

class User < ActiveRecord::Base
  has_many :comments

  accepts_nested_attributes_for :comments
end


class Comment < ActiveRecord::Base
  belongs_to :user
end


u = User.new
# create a new comment via nested attributes
u.update comments_attributes: [{body: "hi"}]
# => true 

# update the comment via nested attributes
u.update comments_attributes: [{id: 1, body: "hi"}]
# => true 

class Comment < ActiveRecord::Base
  belongs_to :user
  
  # add some validation which fails
  validates :body, numericality: true
end

# update the comment via nested attributes
u.update comments_attributes: [{id: 1, body: "hi"}]
# => true 

# It saved? But our comment is invalid...
# We didn't change the comment, so Rails decides we don't need to run validations

# update the comment via nested attributes
u.update comments_attributes: [{id: 1, body: "updated"}]
# => false 

# This is the behavior I would expect in both cases