Room database with one-to-one relation like Address, City and State

Question

I looked in the android documentation for an answer to my question, but I couldn't find it. To create a recyclerview using the information contained in these classes, how can I get a list of this information in Room

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = City::class,
            parentColumns = arrayOf("id"),
            childColumns = arrayOf("cityfk"),
            onDelete = ForeignKey.NO_ACTION
        )
    ]
)
data class Address(
    @PrimaryKey
    @ColumnInfo
    var id: Long = 0
) : Serializable {

    @ColumnInfo
    var name: String = ""

    @ColumnInfo(index = true)
    var cityfk: Long = 0

}

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = State::class,
            parentColumns = arrayOf("id"),
            childColumns = arrayOf("statefk"),
            onDelete = ForeignKey.NO_ACTION
        )
    ]
)
data class City(
    @PrimaryKey
    @ColumnInfo
    var id: Long = 0
) : Serializable {

    @ColumnInfo
    var name: String = ""

    @ColumnInfo(index = true)
    var statefk: Long = 0
}

@Entity
data class State(
    @PrimaryKey
    @ColumnInfo
    var id: Long = 0
) : Serializable {

    @ColumnInfo
    var name: String = ""

}

How can I get a list of addresses listing the classes?

How to get a result like this in ANSI SQL:

select     ADDRESS.NAME ADDRESS
         , CITY.NAME CITY
         , STATE.NAME STATE

from       ADDRESS

join       CITY
on         CITY.ID = ADDRES.CITYFK

join       STATE
on         STATE.ID = CITY.STATEFK

Answer 1

You would typically have a POJO to represent the combined data. You can then either have a field/variable for the extracted columns noting that values are matched to the liked named variable.

You can use @Embedded to include an entity in it's entirety so in theory embed Address City and State.

• see variable/column name issues

You can use @Embedded along with @Relation for the child (children) BUT not for grandchildren (e.g. State). You would need an underlying City with State POJO where City is embedded and State is related by an @Relation.

• variable/column names are not an issue when using @Relation as room builds underlying queries from the parent.

Variable/Column name issues

Room maps columns to variable according to variable names. So there will be issues with id's and name columns if using the simpler @Embedded for all three entities.

• I would suggest always using unique names e.g. addressId, cityId, StateId, (at least for the column names e.g. @ColumnInfo(name = "addressId")) but simpler to just have var addressid.

• An alternative is the use the @Embedded(prefix = "the_prefix") on some, this tells room to match the variable to column name with the prefix so you need to use AS in the SQL. Obviously the_prefix would be changed to suit.

The Dao's

if using @Embedded with @Relation then you simply need to get the parent so

@Query("SELECT * FROM address")
fun getAddressWithCityAndWithState(): List<AddressWithCityAndWithState>

• where AddressWithCityAndWithState is the POJO that has the Address @Embedded and the CityWithState with @Relation.

You would also need the accompanying CityWithState POJO with City @Embedded and State with @Relation.

If Embedding Address, City and State with City having a prefix of "city_" and state having a prefix of "state_" then you would use something like :-

@Query("SELECT address.*, city.id AS city_id, city.name AS city_name, state.id AS state_id, state.name AS state_name FROM address JOIN city ON address.cityfk = city.it JOIN state ON city.statefk = state.id")
fun getAddressWithCityAndWithState(): List<AddressWithCityAndWithState>

• where AddressWithCityAndWithState is the POJO that has Address, City and State @Embedded

Note the above is in-principle.

Working Example

The following is a working example based upon

• a) renaming the columns to avoid ambiguity and
• b) using @Embedded of all three classes in the POJO AddressWithCityWithState

First changes to the Address, City and State to rename the columns :-

Address :-

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = City::class,
            parentColumns = arrayOf("city_id"), //<<<<<<<<<< CHANGED
            childColumns = arrayOf("cityfk"),
            onDelete = ForeignKey.NO_ACTION
        )
    ]
)
data class Address(
    @PrimaryKey
    @ColumnInfo(name ="address_id") //<<<<<<<<<< ADDED name
    var id: Long = 0
) : Serializable {

    @ColumnInfo(name = "address_name") //<<<<<<<<<< ADDDED name
    var name: String = ""

    @ColumnInfo(index = true)
    var cityfk: Long = 0
}

City :-

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = State::class,
            parentColumns = arrayOf("state_id"), //<<<<<<<<<< changed
            childColumns = arrayOf("statefk"),
            onDelete = ForeignKey.NO_ACTION
        )
    ]
)
data class City(
    @PrimaryKey
    @ColumnInfo(name = "city_id") // <<<<<<<<<< ADDED name
    var id: Long = 0
) : Serializable {

    @ColumnInfo(name = "city_name") //<<<<<<<<<< ADDED name
    var name: String = ""

    @ColumnInfo(index = true)
    var statefk: Long = 0
}

State :-

@Entity
data class State(
    @PrimaryKey
    @ColumnInfo(name = "state_id") // ADDED name
    var id: Long = 0
) : Serializable {

    @ColumnInfo(name = "state_name") // ADDED name
    var name: String = ""
}

Next the POJO AddressWithCityWithState :-

data class AddressWithCityWithState (
    @Embedded
    val address: Address,
    @Embedded
    val city: City,
    @Embedded
    val state: State
)

• due to unique column names no prefix = ? required

A suitable DAO :-

@Query("SELECT * FROM address JOIN city on address.cityfk = city.city_id JOIN state ON city.statefk = state.state_id")
    fun getAllAddressesWithCityAndWithState(): List<AddressWithCityWithState>

• simplified due to column renaming so * instead AS clauses for ambiguous column names

Using the above :-

    allDao = db.getAllDao()

    var state = State()
    state.name = "State1"
    var stateid = allDao.insert(state)
    var city = City()
    city.name = "City1"
    city.statefk = stateid
    var cityid = allDao.insert(city)
    var address = Address()
    address.name = "Address1"
    address.cityfk = cityid
    allDao.insert(address)

    for(awcws: AddressWithCityWithState in allDao.getAllAddressesWithCityAndWithState()) {
        Log.d("DBINFO","${awcws.address.name}, ${awcws.city.name}, ${awcws.state.name}")
    }

The result in the log being :-

2021-11-22 07:43:28.574 D/DBINFO: Address1, City1, State1

Other working examples (without changing column names)

Without any changes to the Entities (Address, city and state). Here are working examples of the other options.

1- Get full address as a single string, all that is required is the query such as :-

@Query("SELECT address.name||','||city.name||','||state.name AS fullAddress FROM address JOIN city ON address.cityfk = city.id JOIN state ON city.statefk = state.id ")
fun getAddressesAsStrings(): List<String>

• of course not much use say for a drop down selector as you can't ascertain where in the database the rows came from.

2 - Basic POJO with unambiguous column names

The POJO :-

data class AddressWithCityWithState(
    var address_id: Long,
    var address_name: String,
    var city_id: Long,
    var city_name: String,
    var state_id: Long,
    var state_name: String
)

The query :-

/*
* Returns multiple columns renamed using AS clause to disambiguate
* requires POJO with matching column names
* */
@Query("SELECT " +
        "address.id AS address_id, address.name AS address_name, " +
        "city.id AS city_id, city.name AS city_name, " +
        "state.id AS state_id, state.name AS state_name " +
        "FROM address JOIN city ON address.cityfk = city.id JOIN state ON city.statefk = state.id")
fun getAddressesWithCityAndStateViaBasicPOJO(): List<AddressWithCityWithState>

3- POJO using EMBEDS

The POJO :-

data class AddressWithCityWithStateViaEmbeds(
    @Embedded
    var address: Address,
    @Embedded(prefix = cityPrefix)
    var city: City,
    @Embedded(prefix = statePrefix)
    var state: State
)  {
    companion object {
        const val cityPrefix = "city_"
        const val statePrefix = "state_"
    }
}

The query :-

/*
*   Returns multiple columns renamed according to the prefix=? coded in the
*   @Embedded annotation
*
 */
@Query("SELECT address.*, " +
        "city.id AS " + AddressWithCityWithStateViaEmbeds.cityPrefix + "id," +
        "city.name AS " + AddressWithCityWithStateViaEmbeds.cityPrefix + "name," +
        "city.statefk AS " + AddressWithCityWithStateViaEmbeds.cityPrefix + "statefk," +
        "state.id AS " + AddressWithCityWithStateViaEmbeds.statePrefix + "id," +
        "state.name AS " + AddressWithCityWithStateViaEmbeds.statePrefix + "name " +
        "FROM address JOIN city ON address.cityfk = city.id JOIN state ON city.statefk = state.id")
fun getAddressesWithCityAndStateViaEmbedPOJO(): List<AddressWithCityWithStateViaEmbeds>

4- POJO's with parent EMBED and child RELATE

The POJO's :-

data class CityWithState(
    @Embedded
    var city: City,
    @Relation(
        entity = State::class,
        parentColumn = "statefk",
        entityColumn = "id"
    )
    var state: State
)

and :-

data class AddressWithCityWithStateViaRelations(
    @Embedded
    var address: Address,
    @Relation(
        entity = City::class, /* NOTE NOT CityWithState which isn't an Entity */
        parentColumn = "cityfk",
        entityColumn = "id"
    )
    var cityWithState: CityWithState
)

and the query :-

@Transaction
@Query("SELECT * FROM address")
fun getAddressesWithCityAndStateViaRelations(): List<AddressWithCityWithStateViaRelations>

• note the use of @Tranaction so the underlying queries, built by Room, are all done within a single database transaction.

Putting the above into use

The following code in an activity uses all 4 to output the same results :-

class MainActivity : AppCompatActivity() {

    lateinit var db: TheDatabase
    lateinit var dao: AllDao
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val TAG: String = "DBINFO"

        db = TheDatabase.getInstance(this)
        dao = db.getAllDao()

        var state = State(1)
        state.name = "State1"
        val state1Id = dao.insert(state)
        state.id = 2
        state.name = "State2"
        val state2Id = dao.insert(state)

        var city = City(10)
        city.name = "City1"
        city.statefk = state1Id
        val city1Id = dao.insert(city)
        city.id = 11
        city.name = "City2"
        city.statefk = state2Id
        val city2Id = dao.insert(city)
        city.id = 12
        city.name = "City3"
        city.statefk = state1Id
        val city3Id = dao.insert(city)

        var address = Address(100)
        address.name = "Address1"
        address.cityfk = city1Id
        dao.insert(address)
        address.id = address.id + 1
        address.name = "Address2"
        address.cityfk = city2Id
        dao.insert(address)
        address.id = address.id + 1
        address.name = "Address3"
        address.cityfk = city3Id

        for (s: String in dao.getAddressesAsStrings()) {
            Log.d(TAG + "STRG", s)
        }
        for (awcws: AddressWithCityWithState in dao.getAddressesWithCityAndStateViaBasicPOJO()) {
            Log.d(TAG + "BASICPOJO", "${awcws.address_name}, ${awcws.city_name}, ${awcws.state_name}")
        }
        for (awcwsve: AddressWithCityWithStateViaEmbeds in dao.getAddressesWithCityAndStateViaEmbedPOJO()) {
            Log.d(TAG + "EMBEDS","${awcwsve.address.name}, ${awcwsve.city.name}, ${awcwsve.state.name}")
        }
        for(awcwsvr: AddressWithCityWithStateViaRelations in dao.getAddressesWithCityAndStateViaRelations()) {
            Log.d(TAG + "MIXED","${awcwsvr.address.name}, ${awcwsvr.cityWithState.city.name}, ${awcwsvr.cityWithState.state.name}")
        }
    }
}

The output to the log being :-

2021-11-22 12:33:54.322 D/DBINFOSTRG: Address1,City1,State1
2021-11-22 12:33:54.322 D/DBINFOSTRG: Address2,City2,State2

2021-11-22 12:33:54.324 D/DBINFOBASICPOJO: Address1, City1, State1
2021-11-22 12:33:54.324 D/DBINFOBASICPOJO: Address2, City2, State2

2021-11-22 12:33:54.326 D/DBINFOEMBEDS: Address1, City1, State1
2021-11-22 12:33:54.326 D/DBINFOEMBEDS: Address2, City2, State2

2021-11-22 12:33:54.332 D/DBINFOMIXED: Address1, City1, State1
2021-11-22 12:33:54.332 D/DBINFOMIXED: Address2, City2, State2