Spree API patches to allow 'checkout'.

order_decorator.rb

#order model decorator

Order.class_eval do
  accepts_nested_attributes_for :line_items, :allow_destroy => true

  def self.build_from_api(user, params)
    order = create
    params[:line_items_attributes].each do |line_item|
      order.add_variant(Spree::Variant.find(line_item[:variant_id]), line_item[:quantity])
    end

    if params.key?(:email)
      password_seed = SecureRandom.base64(28)
      password = (0...20).map{ password_seed[rand(password_seed.length)]  }.join

      user = Spree::User.find_or_create_by_email(params[:email],
                                          :login => params[:email],
                                          :password => password,
                                          :password_confirmation => password)
    end

    order.user = user
    order.email = user.email

    order.build_ship_address(params['ship_address_attributes']) if params.key? 'ship_address_attributes'
    order.build_bill_address(params['bill_address_attributes']) if params.key? 'bill_address_attributes'

    if order.line_items.present? && order.ship_address.present? && order.bill_address.present? && order.valid?
      order.shipping_method = order.available_shipping_methods(:front_end).first

      until order.payment?
        order.next!
      end
    end

    order
  end

end

payments_controller_decorator.rb

#api controller decorator

Spree::Api::V1::PaymentsController.class_eval do
  def create
    # one of my uglier hacks to fix issue where failed
    # payments are reverting to 'checkout' state instead
    # of correctly staying 'failed' due to a ROLLBACK
    # cause unknown (me think's core issues).
    #
    @bad_payments = @order.payments.select &:checkout?
    if @bad_payments.present?
      @bad_payments.each {|p| p.destroy }
      @order.reload
    end

    # build as normal
    @payment = @order.payments.build(params[:payment])
    if @payment.save
      # try to progress order to completed state.
      if @order.next
        @order.reload #gets most uptodate payment_state
      end

      render :create, :status => 201
    else
      invalid_resource!(@payment)
    end
  end
end

1. Post to OrderController with everything except the payment.

2. Then post to Payment