add payment model and its associations

app/models/payment.rb

@@ -0,0 +1,27 @@
+class Payment < ActiveRecord::Base
+  has_many :ticket_purchases
+  belongs_to :user
+  belongs_to :conference
+
+  validates :last4, presence: true
+  validates :authorization_code, presence: true
+  validates :status, presence: true
+  validates :amount, presence: true, numericality: { greater_than: 0 }
+  validates :user_id, presence: true
+  validates :conference_id, presence: true
+
+  enum status: {
+    unpaid: 0,
+    success: 1,
+    failure: 2
+  }
+
+  def self.purchase(gateway_response, user, conference)
+    create(last4: gateway_response[:source][:last4],
+           amount: gateway_response[:amount],
+           status: (gateway_response[:paid] ? 1 : 0),
+           authorization_code: gateway_response[:id],
+           user_id: user.id,
+           conference_id: conference.id)
+  end
+end