Robert Sheldon makes me bite my tongue to prevent making schema quality jokes:
In the previous article in this series, I introduced you to schema validation in MongoDB. I described how you can define validation rules on a collection and how those rules validate document inserts and updates. In this article, I continue the discussion by explaining how to apply validation rules to documents that already exist in a collection. Before you start in on this article, however, I recommend that you first review the previous article for an introduction into schema validation.
The examples in this article demonstrate various concepts for working with schema validation, as it applies to existing documents. I show you how to find documents that conform and don’t conform to the validation rules, as well as how to bypass schema validation when inserting or updating a document. I also show you how to update and delete invalid documents in a collection. Finally, I explain how you can use validation options to override the default schema validation behavior when inserting and updating documents.
Read on to learn more about how you can perform some after-the-fact schema validation.