accepts_nested_attributes_for with find_or_create?

When using :accepts_nested_attributes_for, submitting the id of an existing record will cause ActiveRecord to update the existing record instead of creating a new record. I'm not sure what your markup is like, but try something roughly like this:

<%= text_field_tag "team[player][name]", current_player.name %>
<%= hidden_field_tag "team[player][id]", current_player.id if current_player %>

The Player name will be updated if the id is supplied, but created otherwise.

The approach of defining autosave_associated_record_for_ method is very interesting. I'll certainly use that! However, consider this simpler solution as well.

When you define a hook for autosave associations, the normal code path is skipped and your method is called instead. Thus, you can do this:

class Post < ActiveRecord::Base
  belongs_to :author, :autosave => true
  accepts_nested_attributes_for :author

  # If you need to validate the associated record, you can add a method like this:
  #     validate_associated_record_for_author
  def autosave_associated_records_for_author
    # Find or create the author by name
    if new_author = Author.find_by_name(author.name)
      self.author = new_author
    else
      self.author.save!
    end
  end
end

This code is untested, but it should be pretty much what you need.

A before_validation hook is a good choice: it's a standard mechanism resulting in simpler code than overriding the more obscure autosave_associated_records_for_*.

class Quux < ActiveRecord::Base

  has_and_belongs_to_many :foos
  accepts_nested_attributes_for :foos, reject_if: ->(object){ object[:value].blank? }
  before_validation :find_foos

  def find_foos
    self.foos = self.foos.map do |object|
      Foo.where(value: object.value).first_or_initialize
    end
  end

end

Don't think of it as adding players to teams, think of it as adding memberships to teams. The form doesn't work with the players directly. The Membership model can have a player_name virtual attribute. Behind the scenes this can either look up a player or create one.

class Membership < ActiveRecord::Base
  def player_name
    player && player.name
  end

  def player_name=(name)
    self.player = Player.find_or_create_by_name(name) unless name.blank?
  end
end

And then just add a player_name text field to any Membership form builder.

<%= f.text_field :player_name %>

This way it is not specific to accepts_nested_attributes_for and can be used in any membership form.

Note: With this technique the Player model is created before validation happens. If you don't want this effect then store the player in an instance variable and then save it in a before_save callback.