This is quite an edge case, and appears like a bug in Rails (4.2.6) to me.

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Update: This is now documented on Edgeguides Ruby on Rails Show archive.org snapshot :

If you set the :validate option to true, then associated objects will be validated whenever you save this object. By default, this is false: associated objects will not be validated when this object is saved.

Setup

# post.rb
class Post < ActiveRecord::Base
  has_one :attachment
end

# attachment.rb
class Attachment < ActiveRecord::Base
  belongs_to :post
  validates :title, presence: true
end

Issue

When creating a post with an invalid attachment, I expect the creation to fail because of attachment validation errors. However, the attachment is silently not stored to the database.

post = Post.create!(attachment: Attachment.new)
post.id # => 123
post.attachment.id # => nil

This only happens …

• when relying on autosave. Explicitly setting autosave: true fixes this. Also, adding validate: true to has_one :attachment fixes this.
• with has_one. When it's has_many :attachments, validation errors of the attachment properly cancel the creation of the post.
• when creating both post and attachment at the same time. Updating an already persisted post with an invalid attachment cancels the update.