You just need to know the following:
The store will use it's own proxy if you configured one for this instance and if not he takes the proxy from the model.
So you can easily go with two proxy configurations to enable the multi-CRUD operations on the store and the single-CRUD operations on the Models. Note the the static load method of the Model expects the model id
because it is supposed to load a model by just one Id (yes, composite keys are not supported). You will also have to fetch the model instance in the callback (As you did).
Back to your Username/password problem
You may apply your session Model with a custom 'loadSession' method
loadSession: function(username,password, config) {
config = Ext.apply({}, config);
config = Ext.applyIf(config, {
action: 'read',
username: username,
password: password
});
var operation = new Ext.data.Operation(config),
scope = config.scope || this,
callback;
callback = function(operation) {
var record = null,
success = operation.wasSuccessful();
if (success) {
record = operation.getRecords()[0];
// If the server didn't set the id, do it here
if (!record.hasId()) {
record.setId(username); // take care to apply the write ID here!!!
}
Ext.callback(config.success, scope, [record, operation]);
} else {
Ext.callback(config.failure, scope, [record, operation]);
}
Ext.callback(config.callback, scope, [record, operation, success]);
};
this.getProxy().read(operation, callback, this);
}
Now call this instead of load.