insert! returns differently between models

Hi, so I have two models here, lets say one is Model A and the other one is Model B. When I `insert!` Model A, the function returns its identity, but when I `insert!` Model B, the returned value is nil.

The difference between this model is that Model A is using generated id from mysql, where Model B is using id composed in my application:
```
(defmodel B :b
IModel
(pre-insert [payload]
(assoc payload
:id (.toString (java.util.UUID/randomUUID))))

=> (insert! A {:name "John Doe" :email "john@doe.com"}
#AInstance{:id 1 :name "John Doe" :email "john@doe.com"}

=> (insert! B {:name "John Doe" :email "john@doe.com"}
nil
```
Is this behavior expected?

Start at the insert! entry point and compare its handling of Model A's generated MySQL id with Model B's application-assigned UUID from pre-insert. Reproduce both examples, then trace the return-value path; done means the behavior is explained and the issue's expected result is documented or covered by a regression test.

Written by the indexing model from the issue text.