Add this inside your config on the model:
identifier: {
type: 'uuid'
},
Sencha Touch requires each record across all classes to have a identifier. Basically, that's a class that assigns a string to each record. There's javascript classes in the touch source that generate these. Those classes must declare themselves either unique or not. uuid is the best to use that's included in sencha touch and declares itself as unique (with good reason if you take a look at the math it does based on timestamps!)
The reason you need unique identifiers is so that records are not mixed up with each other, especially when it comes to DOM interactions or saving/loading them via a proxy.