Mongodb - Add Schema for existing collection
You can apply a JSON schema to an existing collection using the collMod command to add a new JSON schema to the collection https://docs.mongodb.com/manual/core/schema-validation/. An example below. However it will only apply to new write operations it will not run against existing documents in the collection.
db.runCommand( { collMod: "contacts", validator: { $jsonSchema: { bsonType: "object", required: [ "phone", "name" ], properties: { phone: { bsonType: "string", description: "must be a string and is required" }, name: { bsonType: "string", description: "must be a string and is required" } } } }, validationLevel: "moderate"} )