Steps to reproduce

• A Customer model has accepts_nested_attributes_for :phones and has_many :phones.
• The Phone model has belongs_to :customer

When creating a customer with phone attributes, the validations prevent the number (and customer) from being created.

Please see both of these repos:

• Rails 4.2.6
• Rails 5.0.0.rc1

Rails 4.2.6 works without an issue.

If I disable validations in the controller when saving the customer (by changing @customer.save to @customer.save(validate: false)), both models save and work as expected

Expected behavior

The customer should be saved. The phone should also be saved linked to the customer

Actual behavior

The customer and phone fails to save and throws a validation error

System configuration

Rails version: 5.0.0.rc1

Ruby version: 2.3.1