Develop Reference

Handling Entities and Pojos

Still new to Room and while most of the tutorials I've found are related to simple table and CRUD operations I am stuck on evolving this.
Let's take this sample structures.
Users entity
#Entity(tableName = "users")
public class UsersEntity{
#PrimaryKey(autoGenerate = true)
private long id;
#NonNull
private String name;
#NonNull
private long roleId;
}
Roles entity
#Entity(tableName = "roles")
public class RolesEntity{
#PrimaryKey(autoGenerate = true)
private long id;
#NonNull
private String name;
}
First question: Should Entity objects be extended to also replace POJO? Or have Entities and POJO as separate classes?
Extending from the Room setup, the way I would see User POJO is:
public class User{
private long id;
private Role role;
}
Basically this setup should work both if the User would come as a json response from a web service or entered by the user in the app's input fields.
However, this raises the second question: how to insert and retrieve user info?.
Inserting seems possible as there could be something like
userDAO.insertRole(userId)
But how can I get the Role object of User by using Room and the userDAO?
I find inappropriate to do something like:
user = userDao.getUser(userId)
user.setRole(roleDao.getRole(user.getRoleId)
Third question: it seems to be a good practice to have the table columns with _ (eg. role_id) but in java roleId is recommended as class property. If the result of a #Query for instance select role_id from... and the the POJO with roleId field, will fail so the query needs to be select role_id as roleId... to get it work. Is it a good practice to use camel case in table/column names in sqlite?

What you intend as POJO, probably can be seen as a kind of a view model. In general it is a bad idea to unify/link entities and pojos because you are just making a long wider visibilty/scope for the entity, where it is not necessary and can lead to potential problems.
Say you have some client which requires some different visualization of the data, for instance imagine you have a website which exposes a vehicle data and you have implemented everything using metric system, so for distance you have km, for speed km/h and so on. Now your company gains a huge client from UK, and they want you to provide them the data in imperial format. What to do now? Probably implement a deserilization/conversion process which takes the values and converts them according to the context of the user (whether they are using metric or imperial system). What could possibly go wrong if your entity and view model objects are basically the same? Really bad stuff. You have really tight coupling of things, you should implement different getters for serialization for client, for db..it can become a mess.
Instead if you separate the two, you will have your entity which takes care of working with the database, which is standard procedure with small coefficient of variability, and on the other side you will have the view model which is very likely to require frequent modification, after all it is expected, since it is the interface to the final user.

Android Room Database Nested Object Reference for array of objects

I have a class structure with
Class x {
int a;
int b;
Y[] yList;
}
Class Y {
int m;
int n;
}
I am using data binding, Room and Dagger. I am getting compilation error as 'cannot find class DatabindingComponent', possibly because Room doesn't allow persistent of Nested objects. To enable I used #Embedded annotation but still getting same error. But if I use #Ignore annotation indicating Room that do not process this field; compilation is happening successfully.
How to reference nested array of objects for Room Database without foreign key?
Please refer to
https://developer.android.com/topic/libraries/architecture/room.html#no-object-references
https://developer.android.com/reference/android/arch/persistence/room/Ignore.html
https://developer.android.com/reference/android/arch/persistence/room/Embedded.html.

How to reference nested array of objects for Room Database without foreign key?
If you do not want Y to be an #Entity with its own table, the only option that I know of is to use #TypeConverters:
Define two static methods with #TypeConverter that convert Y[] to and from some basic type (e.g., String, by using JSON)
Register the class holding those methods using #TypeConverters somewhere (e.g., on your RoomDatabase subclass)
This works using collection classes (e.g., List, Set). I would guess that it works with Java arrays (Y[]), though I have not tried it.

Couchbase lite on Android - General architecture?

We are building a project using couchbase. On Android, I use couchbase lite. Usually, I've been working with relational databases and because I am new to couchbase I am having trouble finding the "correct" architecture. I do understand the core concepts I think, but all the samples and guides seem to stick to some kind of easy setup where they access the database right in the Activities.
I am more used to having some database abstraction where the business logic only get's to see POJO DTO's that are delivered through a database interface or some DAO or something. So I've now annotated my model classes and started writing a simple OR mapper, but with different types of data, foreign keys etc. this is getting quite time consuming quite fast.
Am I completely missing the point here somehow? I can't imagine everyone doing it this way? I everyone writing methods that convert Documents to POJO model classes for each class seperately? Or using a json parser to do that (But that won't work for foreign keys if I wan't to load them too, does it)?
Sorry for the load of questions, but I feel I am missing something obvious here. Thanks!

Will try answering your questions:
Am I completely missing the point here somehow?
No. You can treat noSQL CB as a persistent distributed object cache. So its not RDBMS. However, DAO pattern perfectly fits into this model...since you are dealing with DTOs/ValueObjects/POJOs on DAO level and on noSQL level.
I can't imagine everyone doing it this way?
I suggest write one universal Couchbase manager class that can persist/retrieve a POJO. Then you can re-use it in your DAOs.
Everyone writing methods that convert Documents to POJO model classes
for each class separately? Or using a json parser to do that (But that
won't work for foreign keys if I wan't to load them too, does it)?
You can have one common code in your Couchbase manager class that does conversion from/to json to POJO. So you work with only POJOs and don't see any json in your application code (outside of Couchbase manager class)
Here is an example of such class:
public class CouchbaseManager<K, V>
{
private final Class<V> valueTypeParameterClass;
#Inject
private CouchbaseClient cbClient;
#Inject
private Gson gson;
public CouchbaseManager(final Class<V> valueClass)
{
this.valueTypeParameterClass = valueClass;
}
public V get(K key)
{
V res = null;
String jsonValue = null;
if (key != null)
{
jsonValue = (String) cbClient.get(key);
if (jsonValue != null)
{
res = gson.fromJson(jsonValue, valueTypeParameterClass);
}
}
return res;
}
public void put(K key, V value)
{
int ttl = 0;
cbClient.set(key, ttl, gson.toJson(value, valueTypeParameterClass));
}
}
Then in your DAO code you create instance of CouchbaseManager for each type:
CouchbaseManager<String,Customer> cbmCustomer = new CouchbaseManager<String,Customer>(Customer.class);
CouchbaseManager<String,Account> cbmAccount = new CouchbaseManager<String,Account>(Account.class);
// and so on for other POJOs you have.
// then get/put operations look simple
Customer cust = cbmCustomer.get("cust-1234");
cust.setName("New Name"); // mutate value
// store changes
cbmCustomer.put(cust.getId(), cust);
Now regarding "foreign keys". Remember its not RDBMS so its up to your code to have notion of a "foreign key". For example a Customer class can have an id of an account:
Customer cust = cbmCustomer.get("cust-1234");
String accId = cust.getAccountId();
//You can load account
Account acc = cbmAccount.get(accId);
So as you can see you are doing it all yourself. I wish it was JPA or JDO implementation/provider for Couchbase (like DataNucleus or Hibernate)
You should really start with your POJO/Document design to try to split your POJO entities into "chunks" of data to get a right balance between coarse vs fine grained POJOs.
Also see this discussion on key/document design considerations.

Defining an ActiveAndroid model without creating a table

I have my app's networking code separated into a separate package. This package contains a POJO model that I'm serializing with Gson (using its #Expose annotation):
public class User {
#Expose
String username;
#Expose
String pathToAvatar;
// etc.
}
My app also is using ActiveAndroid as a database wrapper. I would like to extend the .networking.User model and add a couple more database-specific fields (secretKey or something) in my main logic. It would be nice if the networking code did not know its model was being used in a database. Unfortunately, I'm having to have .networking.User extend the ActiveAndroid Model base class (and having to label each networking variable with #Column). This works, with one problem: I'm getting a dummy User database table that corresponds with the networking code's class, even though it is not annotated with #Table, simply because it derives from Model. The main code's table is created correctly.
Can I prevent ActiveAndroid from creating a table when it finds a certain Model? Alternatively, is there a better way to approach this?

retrofit and orm library throw StackOverflow

I try to use 2 libraries:
square/Retrofit - Rest client
satyan/sugar - db orm
retrofit use gson, so do class
public class Book{
String name;
public Book(String name) {
this.name = name;
}
}
ok, retrofit succesfully get data from server and put in our Book class.
now i want save this data. for use orm need extend parent class
public class Book extends SugarRecord<Book>{
String name;
public Book(String name) {
this.name = name;
}
}
but after extend the parent class, retrofit cannot parse json.
so we get an error:
java.lang.RuntimeException: An error occured while executing doInBackground()
...
Caused by: retrofit.RetrofitError: java.lang.StackOverflowError at retrofit.RestAdapter$RestHandler.invokeRequest(RestAdapter.java:390)
...
Caused by: java.lang.StackOverflowError at com.google.gson.internal.$Gson$Types.resolve($Gson$Types.java:375)
...
how to make friends 2 libraries that they use one object class?
or how to specify the retrofit, so that it did not touch Book's class parent?

Error happens when Gson tries to resolve type information about an object it have to deserialize. It gets into an infinite recursion due to the cyclic reference to your Book class in extends declaration.
However even if Gson could cope with your class, I would not recommend using these libs combination.
You see, what Gson does is much alike to what standard Java serialization does but in JSON format. I mean that Gson takes the internal state of your object and performs its serialization. And when it parses JSON it creates an object with the state specified in this JSON.
If you take a look at SugarRecord class, you'll see that it has a field named "tableName". Thus if you passed your Book object to Gson instance, you'd get
{name: "book name", tableName: "table_book"}.
Moreover, if you got a response from server which is like
{name: "another book name", tableName: "something absolutely unrelated"},
you would get an instance of Book with a state exactly matching what is described in this response. Meaning, with tableName being not equal to what you would like...
You could workaround this issue using exclusion strategies in Gson, but overall you'll get yet another problem.
P.S. After a quick look at SugarRecord class on github I do not understand why it has a type parameter at all. It's even not used really. Thus technically I think you'll be able to combine these 2 libraries, if you skip type parameter in extends declaration making your class look like class Book extends SugarRecod { }, and use an exclusion strategy. Yet, I wouldn't do it myself in practice :).

Your POJO class need a empty constructor :
you shoud add this constructor to your Book class:
public Book(){
}