Migrate unsubscribe keys to the database.

This should reduce a lot of the keys in redis.

db/migrate/20150213174159_create_digest_unsubscribe_keys.rb

@@ -0,0 +1,46 @@
+class CreateDigestUnsubscribeKeys < ActiveRecord::Migration
+  def up
+    create_table :digest_unsubscribe_keys, id: false do |t|
+      t.string :key, limit: 64, null: false
+      t.integer :user_id, null: false
+      t.timestamps
+    end
+    execute "ALTER TABLE digest_unsubscribe_keys ADD PRIMARY KEY (key)"
+    add_index :digest_unsubscribe_keys, :created_at
+
+    migrate_redis_keys
+  end
+
+  # It is slightly odd to migrate from redis to postgres; I imagine a lot
+  # could fail, so if anything does we just rescue
+  def migrate_redis_keys
+    return if Rails.env.test?
+
+    temp_keys = $redis.keys('temporary_key:*')
+    if temp_keys.present?
+      temp_keys.map! do |key|
+        user_id = $redis.get(key).to_i
+        ttl = $redis.ttl(key).to_i
+
+        if ttl > 0
+          ttl = "'#{ttl.seconds.ago.strftime('%Y-%m-%d %H:%M:%S')}'"
+        else
+          ttl = "CURRENT_TIMESTAMP"
+        end
+        $redis.del(key)
+        key.gsub!('temporary_key:', '')
+        user_id ? "('#{key}', #{user_id}, #{ttl}, #{ttl})" : nil
+      end
+      temp_keys.compact!
+      if temp_keys.present?
+        execute "INSERT INTO digest_unsubscribe_keys (key, user_id, created_at, updated_at) VALUES #{temp_keys.join(', ')}"
+      end
+    end
+  rescue
+    # If anything goes wrong, continue with other migrations
+  end
+
+  def down
+    drop_table :digest_unsubscribe_keys
+  end
+end