Custom Validation

[cddr]

As a developer, I'd like to be able to add custom validation rules that are checked as part of the transaction so that I can be sure of the integrity of persisted data.

Functional Requirements

• [ ] The definition given to defentity should support a new option :validation.
• [ ] This should be a function that accepts two arguments. The first is a datomic database instance as-of the start of the transaction. The second is the input data that is to be applied relating to this entity. The function should return a two element tuple, the first of which indicates whether it is ok to proceed with the transaction, and the second indicates any error messages if it is not ok.
• [ ] This function should be called during POST/PUT/PATCH operations to decide whether it is ok to process the request. If the transaction fails, the request handler should return with a 422 containing any error messages in the response body

Non-Functional Requirements

• [ ] To avoid race conditions, this should probably be implemented as a database function through which all database writes are performed
• [ ] There are a whole bunch of existing clojure libraries for validating user input. It would be nice if our implementation did not prevent them from using whatever one they want.