How to Dynamically change sync function in sync gateway couchbase

Question

Is there a way I can dynamically change sync function for eg. Lets ssy my documents have a field ID and I want to get documents belonging to a particular ID so ID is my variable here. eg. below is a sync function for ID=4

"sync":
    function (doc) {
        if(doc.ID==4){
            channel (doc.channels);
        }
        else{
            throw({forbidden: "Missing required properties"});
        }
    }, 

Now This will only work for ID=4. How Can I make my sync function dynamic. Is there a way I can supply arguments to my sync function?

EDIT 1 Added Use Case

Ok so my use case is like this.I have an app in which when a user logs in I need to get user specific data from CouchBase Server to CouchBase lite. In My CouchBase Server I have 20000 documents and for each user there are 5 documents so I have (20000/5) 4000 users. So When a user logs in to my app my CouchBase server should send only 5 documents which are related to that user and not all 20000 documents

EDIT 2

This is how I have implemented the replication

private URL createSyncURL(boolean isEncrypted){
    URL syncURL = null;
    String host = "http://172.16.25.108";
    String port = "4986";
    String dbName = "sync_gateway";
    try {
        //syncURL = new URL("http://127.0.0.1   :4986/sync_gateway");
        syncURL = new URL(host + ":" + port + "/" + dbName);
    } catch (Exception me) {
        me.printStackTrace();
    }
    Log.d(syncURL.toString(),"URL");
    return syncURL;
    }
    private void startReplications() throws CouchbaseLiteException {
    Log.d(TAG, "");
    Replication pull = database.createPullReplication(this.createSyncURL(false));
    Replication push = database.createPushReplication(this.createSyncURL(false));
    Authenticator authenticator = AuthenticatorFactory.createBasicAuthenticator("an", "1234");
    pull.setAuthenticator(authenticator);
    //push.setAuthenticator(authenticator);
    List<String> channels1 = new ArrayList<String>();
    channels1.add("u1");
    pull.setChannels(channels1);
    pull.setContinuous(true);
       // push.setContinuous(true);
    pull.start();
    //push.start();
    if(!push.isRunning()){
        Log.d(TAG, "MyBad");
    }
    /*if(!push.isRunning()) {
        Log.d(TAG, "Replication is not running due to " +push.getLastError().getMessage());
        Log.d(TAG, "Replication is not running due to " +push.getLastError().getCause());
        Log.d(TAG, "Replication is not running due to " +push.getLastError().getStackTrace());
        Log.d(TAG, "Replication is not running due to " +push.getLastError().toString());
    }*/
    }

Answer 1

The easiest way to achieve this is to qualify each user to one channel, named like the user, and to give to a document the channel (= user) names of all users for whom this document is relevant (maybe just one channel name per document, but that's completely up to you).

So with the standard sync function (without any if condition), if your config.json contains

        "users": {
            "u1": {
                "admin_channels": ["u1"],
                "password": "abracadabra"
            },
            "u2": {
                "admin_channels": ["u2"],
                "password": "simsalabim"
        ...

and you have documents having

{"channels": "u1",...

{"channels": "u2",...

{"channels": ["u1", "u2"],...

then the first will be transferred to u1, the second to u2, and the third to both of them. You don't need to make your channel names identical to the user name, but for this scenario it's the easiest way to go.

The programmatic assignment of channels to users can be done via the Sync Gateway Admin REST API, see http://developer.couchbase.com/documentation/mobile/1.2/develop/references/sync-gateway/admin-rest-api/user-admin/post-user/index.html. (Note that the Admin API should run on a port that is opened only to the local server where CB runs, not to the public.)