When are property validations run in Google App Engine (GAE)?

Question

So I was reading the following documentation on defining your own property types in GAE. I noticed that I could also include a .validate() method when extending a new Property. This validate method will be called "when an assignment is made to a property to make sure that it is compatible with your assigned attributes". Fair enough, but when exactly is that?

My question is, when exactly is this validate method called? Specifically, is it called before or after it is put? If I create this entity in a transaction, is validate called within the transaction or before the transaction?

I am aware that optimally, every Property should be "self contained" or at most, it should only deal with the state of the entity is resides in. But, what would happen if you performed a Query in the validate method? Would it blow up if you did a Query within validate that was in a different entity group than your current transactions entity group?

Answer 1

Before put, and during the transaction, respectively (it may abort the transaction if validation fails of course). "When an assignment is made" to a property of your entity is when you write theentity.theproperty = somevalue (or when you perform it implicitly).

I believe that queries of unrelated entities during a transaction (in validate or otherwise) are non-transactional (and thus very iffy practice), but not forbidden -- but on this last point I'm not sure.