How to create Entity and data classes by ROOM in android

Question

How to create Entity and data classes by ROOM in android?

I have JSON structure:

data class ListResponse(val item: ListItem)

data class ListItem(
    @SerializedName("id")
    val id: List<CheckUnCheckItem>
)

data class CheckUnCheckItem(
    @SerializedName("check")
    val check: CheckItem,

    @SerializedName("unCheck")
    val UnCheck: UnCheckItem
)

data class CheckItem(
    @SerializedName("url")
    val url: String,

    @SerializedName("text")
    val text: String,

    @SerializedName("color")
    val color: String
)

data class UnCheckItem(
    @SerializedName("url")
    val urlUnCheck: String,

    @SerializedName("text")
    val textUnCheck: String,

    @SerializedName("color")
    val colorUnCheck: String
)

But How can I create such ROOM Entity?

Do I need to use @TypeConverter?

@Entity(tableName = TABLE_NAME)
data class ListEntity(
    @PrimaryKey @SerializedName("id")
    val id: CheckUnCheckItem,

    @SerializedName("check")
    val check: CheckItem,

    @SerializedName("unCheck")
    val unCheck: UnCheckItem,

    @SerializedName("url")
    val url: String,

    @SerializedName("text")
    val text: String,

    @SerializedName("size")
    val size: String
){
    companion object{
        const val TABLE_NAME = "db_table"
    }

    class RoomTypeConverters{
        @TypeConverter
        fun convertCheckItemListToJSONString(checkList: CheckItem): String = Gson().toJson(checkList)
        @TypeConverter
        fun convertJSONStringToCheckItemList(jsonString: String): CheckItem = Gson().fromJson(jsonString,CheckItem::class.java)

    }
}

is my data and entity classes are correct? Do I need class witch extends RoomDatabase? Or better I need to separate db and create for check and uncheck another db?

Answer 1

Or better I need to separate db and create for check and uncheck another db?

As database implies it is able to store data not just one but many. As such a single database is all that would be required. SQLite is a relational database and is designed to store related data. Related data is typically stored in multiple tables. So again a single database will very likely be sufficient.

Do I need to use @TypeConverter? You never actually need Type Converters. However, for any Object, other than those directly handled (e.g. String, Int, Long, Double, Float, ByteArray) then you either need to break these down into such handled objects or have a types converter that will convert the object to and from such an object.

For example, based upon your @Entity annotated ListEntity class then:-

• field id would need a TypeConverter as the type CheckUnCheckItem is not an object type that can be directly handled by Room. So you would need two TypeConverters that could convert from a CheckUncheckItem to and from a type that can be handled by Room.
• fields check and uncheck would need two TypeConverters (and it looks as though the Type Converters you have coded will handle the conversion).
• fields url,text and size, as they are all String types do not need Type Converters as Room handles strings.

Room has to know about the Type Converters. So you need an @TypeConverters annotation. It's placement defines the scope. Using the annotation to preced the @Database annotation has the most far reaching scope.

Do I need class witch extends RoomDatabase? Yes. However it has to be an abstract class and should have an abstract function to retrieve an instance of each @Dao annotated interface (or abstract class, in which case the functions have to be abstract, there is no need for an abstract class with Kotlin as functions in an interface can have bodies)).

This class should be annotated with the @Database annotation, the entities parameter of the annotation should include the list of classes for each each table (@Entity annotated class). e.g.

@TypeConverters(value = [ListEntity.RoomTypeConverters::class])
@Database(entities = [ListEntity::class], exportSchema = false, version = 1)
abstract class TheDatabase: RoomDatabase(){

}

• However, using the above along with your classes results in a build error as per:-

• ListEntity.java:11: error: Cannot figure out how to save this field into database. You can consider adding a type converter for it. private final a.a.so74708202kotlinroomentitydesign.CheckUnCheckItem id = null;

  • as explained CheckUnCheckItem cannot be handled by Room.

Amending the RoomTypeConverters class to be:-

class RoomTypeConverters{
    @TypeConverter
    fun convertItemListToJSONString(invoiceList: Item): String = Gson().toJson(invoiceList)
    @TypeConverter
    fun convertJSONStringToItemList(jsonString: String): Item = Gson().fromJson(jsonString,Item::class.java)
    @TypeConverter
    fun convertCheckUnCheckItemToJSONString(cuc: CheckUnCheckItem): String = Gson().toJson(cuc)
    @TypeConverter
    fun convertJSONStringToCheckUnCheckItem(jsonString: String): CheckUnCheckItem = Gson().fromJson(jsonString,CheckUnCheckItem::class.java)

}

Resolves the build issue and in theory you have a potentially usable database.

However, you obviously need code to access the database. As such you would very likely want to have. as previously mentioned, an @Dao annotated interface e.g

@Dao
interface TheDAOs {
    @Insert
    fun insert(listEntity: ListEntity): Long
    @Query("SELECT * FROM ${TABLE_NAME}")
    fun getAll(): List<ListEntity>
}

• This will suffice to allow rows to be inserted into the database and for all the rows to be extracted from the database into a List<ListEntity).

• From the built database you need to get an instance of the TheDAOs and thus the @Database annotated class could then be

:-

@TypeConverters(value = [ListEntity.RoomTypeConverters::class])
@Database(entities = [ListEntity::class], exportSchema = false, version = 1)
abstract class TheDatabase: RoomDatabase(){
    abstract fun getTheDAOsInstance(): TheDAOs

}

To demonstrate actual use of the above then consider the following code in an activity:-

class MainActivity : AppCompatActivity() {

    lateinit var roomDBInstance: TheDatabase
    lateinit var theDAOs: TheDAOs
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        roomDBInstance = Room.databaseBuilder(this,TheDatabase::class.java,"The_database_name.db")
            .allowMainThreadQueries() /* NOTE ADDED FOR CONVENIENCE AND BREVITY */
            .build()
        /* Note the database itself does not yet exist, it's creation is delayed until an attempt is made to access it. So:- */
        theDAOs = roomDBInstance.getTheDAOsInstance() /* Still the database is not created/accessed */
        showData(theDAOs.getAll()) /* No data has been added BUT the database will now exist */

        theDAOs.insert(
            ListEntity(
                id =  CheckUnCheckItem(
                    check = Item (
                        url ="URL001",
                        text = "TEXT001",
                        color = "RED"
                    ),
                    unCheck =  Item(
                        url ="URL002",
                        text = "TEXT002",
                        color = "BLUE"
                    )
                ),
                check =  Item(url = "URL003", text ="TEXT003", color ="WHITE"),
                unCheck =  Item(url = "URL004", text = "TEXT004", color = "BLACK"),
                url = "URL005", text = "TEXT005", size = "BIG"
            )
        )
        showData(theDAOs.getAll())
    }

    fun showData(listEntities: List<ListEntity>) {
        for (li in listEntities) {
            Log.d(
                "DBINFO",
                "id is $li.id.check.url${li.id.check.text}.... " +
                        "\n\tcheck is ${li.check.url} .... " +
                        "\n\tuncheck is ${li.unCheck.url} ...." +
                        "\n\turl is ${li.url} text is ${li.text} size is ${li.size}"
            )
        }
    }
}

The output to the log being:-

D/DBINFO: id is ListEntity(id=CheckUnCheckItem(check=Item(url=URL001, text=TEXT001, color=RED), unCheck=Item(url=URL002, text=TEXT002, color=BLUE)), check=Item(url=URL003, text=TEXT003, color=WHITE), unCheck=Item(url=URL004, text=TEXT004, color=BLACK), url=URL005, text=TEXT005, size=BIG).id.check.urlTEXT001.... 
        check is URL003 .... 
        uncheck is URL004 ....
        url is URL005 text is TEXT005 size is BIG

The Database via App Inspection being"-

So finally

is my data and entity classes are correct?

From a database aspect yes, they work after a few amendments. However, I suspect that your classes are probably not what you intended.

---

An Alternative Approach

If this were to be approached from a database perspective and normalised and without bloat and without the need for type converters then consider the following:-

The embedded Item's (uncheck and check) are basically repetition, so could probably be a table (related to the db_table). Hence 2 tables. One for the ListEntity (Alternative) and another for the Items (AlternativeItem) so the 2 @Entity annotated classes could be:-

/* Alternative Approach */
@Entity(
    /* Foreign Keys NOT REQUIRED, they enforce Referential Integrity */
    foreignKeys = [
        ForeignKey(
            entity = AlternativeItem::class,
            parentColumns = ["alternativeItemId"],
            childColumns = ["unCheckIdMap"]
            /* OPTIONAL within a Foreign Key, they help automatically maintain Referential Integrity*/,
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        ),
        ForeignKey(
            entity = AlternativeItem::class,
            parentColumns = ["alternativeItemId"],
            childColumns = ["checkIdMap"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        )
    ]
)
data class Alternative(
    @PrimaryKey
    val id: Long?=null,
    @ColumnInfo(index = true)
    val unCheckIdMap: Long, /* map to the id of the related Item (AlternativeItem) for the uncheck */
    @ColumnInfo(index = true)
    val checkIdMap: Long, /* map to the id of the related Item (AlternativeItem) for the uncheck */
    val url: String,
    val text: String,
    val size: String
)
@Entity
data class AlternativeItem(
    @PrimaryKey
    val alternativeItemId: Long?=null,
    val alternativeItemUrl: String,
    val alternativeItemText: String,
    val alternativeItemColor: String
)

As you would typically want the Alternative along with it's related AlternativeItems then a POJO that caters for the togetherness :-

data class AlternativeWithUncheckAndCheck(
    @Embedded
    val alternative: Alternative,
    @Relation(entity = AlternativeItem::class, parentColumn = "unCheckIdMap", entityColumn = "alternativeItemId")
    val unCheck: AlternativeItem,
    @Relation(entity = AlternativeItem::class, parentColumn = "checkIdMap", entityColumn = "alternativeItemId")
    val check: AlternativeItem
)

There would be a need for some extra functions in the @Dao annotated interface, so :-

@Insert
fun insert(alternative: Alternative): Long
@Insert
fun insert(alternativeItem: AlternativeItem): Long

@Transaction
@Query("")
fun insertAlternativeAndUncheckAndCheck(alternative: Alternative, uncheck: AlternativeItem, check: AlternativeItem): Long {
    var uncheckId = insert(uncheck)
    var checkId = insert(check)
    return insert(Alternative(null,url = alternative.url, text = alternative.text, size = alternative.size, unCheckIdMap = uncheckId, checkIdMap = checkId ))
}

@Transaction
@Query("SELECT * FROM alternative")
fun getAllAlternativesWithRelatedUnCheckAndCheck(): List<AlternativeWithUncheckAndCheck>

• note that the insertAlternativeAndUncheckAndCheck does what it says (note that it is overly simple and could need some enhancements to expand upon the principle)

To demonstrate this, all that is then required is to add the new entities to the entities parameter and to then add some code to the activity.

The amended @Database annotation:-

@Database(entities = [ListEntity::class, /* for the alternative approach */ Alternative::class, AlternativeItem::class], exportSchema = false, version = 1)

The activity code (that caters for both approaches in a similar/equivalanet way of storing and retrieving the data) :-

class MainActivity : AppCompatActivity() {

    lateinit var roomDBInstance: TheDatabase
    lateinit var theDAOs: TheDAOs
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        roomDBInstance = Room.databaseBuilder(this,TheDatabase::class.java,"The_database_name.db")
            .allowMainThreadQueries() /* NOTE ADDED FOR CONVENIENCE AND BREVITY */
            .build()
        /* Note the database itself does not yet exist, it's creation is delayed until an attempt is made to access it. So:- */
        theDAOs = roomDBInstance.getTheDAOsInstance() /* Still the database is not created/accessed */
        showData(theDAOs.getAll()) /* No data has been added BUT the database will now exist */

        theDAOs.insert(
            ListEntity(
                id =  CheckUnCheckItem(
                    check = Item (
                        url ="URL001",
                        text = "TEXT001",
                        color = "RED"
                    ),
                    unCheck =  Item(
                        url ="URL002",
                        text = "TEXT002",
                        color = "BLUE"
                    )
                ),
                check =  Item(url = "URL003", text ="TEXT003", color ="WHITE"),
                unCheck =  Item(url = "URL004", text = "TEXT004", color = "BLACK"),
                url = "URL005", text = "TEXT005", size = "BIG"
            )
        )
        showData(theDAOs.getAll())


        /* Alternative equivalent */

        theDAOs.insertAlternativeAndUncheckAndCheck(
            Alternative(url = "URL005", size = "BIG", text = "TEXT005", checkIdMap = -1, unCheckIdMap = -1),
            check = AlternativeItem(alternativeItemUrl = "URL001", alternativeItemText = "TEXT001", alternativeItemColor = "RED"),
            uncheck = AlternativeItem(alternativeItemUrl = "URL002", alternativeItemText = "TEXT002", alternativeItemColor = "BLUE" )
        )
        showAlternativeData(theDAOs.getAllAlternativesWithRelatedUnCheckAndCheck())
    }

    fun showData(listEntities: List<ListEntity>) {
        for (li in listEntities) {
            Log.d(
                "DBINFO",
                "id is $li.id.check.url${li.id.check.text}.... " +
                        "\n\tcheck is ${li.check.url} .... " +
                        "\n\tuncheck is ${li.unCheck.url} ...." +
                        "\n\turl is ${li.url} text is ${li.text} size is ${li.size}"
            )
        }
    }

    fun showAlternativeData(listAlternatives: List<AlternativeWithUncheckAndCheck>) {
        for (la in listAlternatives) {
            Log.d("DBALTINFO",
            "id is ${la.alternative.id} URL is ${la.alternative.url} TEXT is ${la.alternative.text} SIZE is ${la.alternative.size} " +
                    "\n\t UNCHECK id is ${la.unCheck.alternativeItemId} url is ${la.unCheck.alternativeItemUrl} text is ${la.unCheck.alternativeItemText} color is ${la.unCheck.alternativeItemColor}" +
                    "\n\t CHECK id is ${la.check.alternativeItemId} url is ${la.check.alternativeItemUrl} text is ${la.check.alternativeItemText} color is ${la.check.alternativeItemColor}")
        }
    }
}

• Note that the Alternative code is probably more along the lines of what you probably want according to the interpretation of the shown JSON.

When run then the result is now:-

D/DBINFO: id is ListEntity(id=CheckUnCheckItem(check=Item(url=URL001, text=TEXT001, color=RED), unCheck=Item(url=URL002, text=TEXT002, color=BLUE)), check=Item(url=URL003, text=TEXT003, color=WHITE), unCheck=Item(url=URL004, text=TEXT004, color=BLACK), url=URL005, text=TEXT005, size=BIG).id.check.urlTEXT001.... 
        check is URL003 .... 
        uncheck is URL004 ....
        url is URL005 text is TEXT005 size is BIG
        
        
D/DBALTINFO: id is 1 URL is URL005 TEXT is TEXT005 SIZE is BIG 
         UNCHECK id is 1 url is URL002 text is TEXT002 color is BLUE
         CHECK id is 2 url is URL001 text is TEXT001 color is RED

• it is suspected that BLACK/WHITE or RED/BLUE is superfluous in your interpretationof the JSON to data classes (and hence excluded in the alternative).

The database, via App Inspection (in regards to the alternative approach) is:-

and :-

i.e. only the actual data is store the BLOAT (field/type descriptions, separators, enclosing data) is not stored thus

• the database will hold more data in less space.
• the handling of the data will thus be more efficient (e.g. a buffer can hold more actual data instead of BLOAT).
• querying the data such as for example searching for all BLUE's is directly a search for that, whilst with converted data you may have issues distinguishing between BLOAT and actual data

However, the negative, is that more code and thought is required.

Note this answer is intended to deal with the basic principles and is most certainly not fully comprehensive.