Android Jetpack Room One to Many Query Where - android-room

Question

I have two tables of Contacts and Messages
Contacts
#Entity
data class Contacts(
#PrimaryKey
var id: String,
var user: String,
var friend: String,
var recentMessage: String?,
var recentDate: Date,
var createDate: Date
)
2.Messages
#Entity
data class Messages(
#PrimaryKey
var id: String,
var contactsId: String,
var user: String,
var friend: String,
var text: String = "",
var createDate: Date,
)
One-to-one query entity is defined
data class ContactsMessages(
#Embedded val contacts: Contacts,
#Relation(parentColumn = "id", entityColumn = "contactsId")
val messages: Messages?
)
My Dao query method works very well, but only the oldest one can be queried. What if the latest one is queried?
#Transaction
#Query("SELECT * FROM Contacts where user = :user order by recentDate desc")
fun contactsMessageList(vararg user: String): Flow<List<ContactsMessages>>
I can easily change it to one-to-many. If it is one-to-many, how do I sort the side with the more? If I control the number of the side with the more?

Answer 1

I can easily change it to one-to-many. If it is one-to-many, how do I sort the side with the more? If I control the number of the side with the more?
The problem with #Relation is that if you let Room build the result, it does so by getting the parent(s) (the Embedded object) and then it gets ALL children (#Relation objects) via a query that it builds that you have no control over.
One way around this is to get the parent(s) and then get the single child separately. This could be, for example by utilising the following Dao's :-
#Query("SELECT * FROM contacts WHERE user=:user")
abstract fun getContactsByUser(vararg user:String): List<Contacts>
gets the respective Contacts
and :-
#Query("SELECT * FROM messages WHERE contactsId=:contactsId ORDER BY createDate ASC LIMIT 1")
abstract fun orderedMessage(contactsId: String): Messages
gets the respective Message for a single Contacts
and then using the two queries separately using a function for this e.g. :-
fun getOrderedContactsMessages(vararg user: String): List<ContactsMessages> {
var rv = ArrayList<ContactsMessages>()
for(u: String in user) {
for (c: Contacts in getContactsByUser(u))
rv.add(ContactsMessages(c,orderedMessage(c.id)))
}
return rv
}
It should be noted that instead of being in an interface the above is coded in an abstract class and hence the need for abstract fun .... for the functions that would have been abstract if included in an interface.
Working Example
Using the above (noting that for brevity and convenience, run on the main thread, and strings have been used for dates) in conjunction with:-
dao.insert(Contacts("FredID","Fred","Fred's friend","recent message","2021-10-01","2021-01-01"))
dao.insert(Contacts("MaryID","Mary","Mary's friend","recent message","2021-09-30","2021-01-01"))
dao.insert(Contacts("JaneID","Jane","Jane's friend","recent message","2021-09-29","2021-01-01"))
dao.insert(Messages("MSG99","FredID","Fred","This is message 99","2021-10-01","2021-10-01"))
dao.insert(Messages("MSG98","FredID","Fred","This is message 98","2021-09-01","2021-09-01"))
dao.insert(Messages("MSG97","FredID","Fred","This is message 97","2021-08-01","2021-08-01"))
dao.insert(Messages("MSG89","MaryID","Mary","This is message 89","2021-09-30","2021-09-30"))
dao.insert(Messages("MSG88","MaryID","Mary","This is message 88","2021-09-01","2021-09-01"))
dao.insert(Messages("MSG87","MaryID","Mary","This is message 87","2021-08-01","2021-08-01"))
dao.insert(Messages("MSG79","JaneID","Jane","This is message 79","2021-09-29","2021-09-29"))
dao.insert(Messages("MSG78","JaneID","Jane","This is message 78","2021-09-01","2021-09-01"))
dao.insert(Messages("MSG77","JaneID","Jane","This is message 77","2021-08-01","2021-08-01"))
for(cm: ContactsMessages in dao.getOrderedContactsMessages("Fred","Mary")) {
Log.d("MYINFO","ID is ${cm.contacts.id} recent message is ${cm.contacts.recentMessage} ordered message is ${cm.messages!!.text}")
}
Then the result written to the log is :-
D/MYINFO: ID is FredID recent message is recent message ordered message is 2021-08-01
D/MYINFO: ID is MaryID recent message is recent message ordered message is 2021-08-01
if orderedMessage is changed to DESC instead of ASC then :-
D/MYINFO: ID is FredID recent message is recent message ordered message is 2021-10-01
D/MYINFO: ID is MaryID recent message is recent message ordered message is 2021-09-30
efficiency wise, there would be ways to do this via a single query but the SQL would be more complex and it would utilise a different POJO with Messages embedded, which would then require the like named columns to be disambiguated.