更新Rails 4和PostgreSQL json列中的单个键/值对？

I have a rather large json document that I have to store in a field for each Evaluation instance in my app. The over time, certain actions on the app will require me to change various key/value pairs in the document. The Rails 4 and PostgreSQL json data type seems ideal for this problem, but I cannot get changes to commit to the database. I have a rather large json document that I have