Multiple fields have the same columnName Android Room - mysql

Question

I have 3 tables ruser, accounts, accountgroup. Each one has a same column called rsuerId.
I created a POJO class with 3 Embedded objects as below.
class GroupChatItem(
#Embedded
val rUserDto: RUserDto,
#Embedded
val account: AccountDto,
#Embedded
val accountGroup: AccountGroupDto
)
Now, i want to make a query that fetches a GroupChatItem with a given rUserId and accountGroupId like the following.
#Query("""
Select ruser.*, accounts.*, accountgroup.*
from ruser
inner join accounts on accounts.rUserId = ruser.rUserId and accounts.active = 1
inner join accountgroup on accountgroup.rUserId = :rUserId and accountGroup.accountGroupId = :accountGroupId
where ruser.rUserId = :rUserId
""")
suspend fun getGroupChatItem(rUserId: Long, accountGroupId: Int): GroupChatItem
Unfortunately i get the following error.
Multiple fields have the same columnName: rUserId. Field names: rUserDto > rUserId, account > rUserId, accountGroup > rUserId.
I have tried to add a prefix to each embedded object but i get also an error. I dont want to retrieve columns one-by-one because there are many of them.
Is there anything that i missed...??
Thank you

Answer 1

Alternatively you can use the prefix attribute of the Embedded anotation:
class GroupChatItem(
#Embedded(prefix = "user_")
val rUserDto: RUserDto,
#Embedded(prefix = "acc_")
val account: AccountDto,
#Embedded(prefix = "accgr_")
val accountGroup: AccountGroupDto
)
and then alias all the columns of each entity in your SQL query.
I think the prefix attribute is s recent update but I am not sure

Answer 2

I don't believe you have any option other than to have/use :-
a) have distinct columns names across the tables that are to be included in joins (then there is no need to prefix the column names),
or
b) to rename the columns using AS when extracting the values along with a prefix when embedding the entity ensuring that the names match.
I believe that a) would be the simpler option as there is a reduction in the chance of inadvertently using the wrong column name.
As I understand it, the column names have to match for Room to be able to know how to be able to copy a value from the underlying result set, which has no indication of what table a value came from to the value in the returned object or objects.
This is an example of the generated code of a similar scenario 3 embedded entities (User, Office and Places) where some of the column names are the same. They each have and id column and User and Places both have a columns named name.
#Override
public UserOfficePlacesCombined getAllUserOfficePlacesCombined() {
final String _sql = "SELECT user.id AS userid, user.name AS username, office.id AS officeid, office.address AS officeaddress, places.id AS placesid, places.name AS placesname FROM User JOIN Office ON User.id = Office.id JOIN Places ON User.id = Places.id";
final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 0);
__db.assertNotSuspendingTransaction();
final Cursor _cursor = DBUtil.query(__db, _statement, false, null);
try {
final int _cursorIndexOfId = CursorUtil.getColumnIndexOrThrow(_cursor, "userid");
final int _cursorIndexOfName = CursorUtil.getColumnIndexOrThrow(_cursor, "username");
final int _cursorIndexOfId_1 = CursorUtil.getColumnIndexOrThrow(_cursor, "officeid");
final int _cursorIndexOfAddress = CursorUtil.getColumnIndexOrThrow(_cursor, "officeaddress");
final int _cursorIndexOfId_2 = CursorUtil.getColumnIndexOrThrow(_cursor, "placesid");
final int _cursorIndexOfName_1 = CursorUtil.getColumnIndexOrThrow(_cursor, "placesname");
final UserOfficePlacesCombined _result;
if(_cursor.moveToFirst()) {
final User _tmpUser;
if (! (_cursor.isNull(_cursorIndexOfId) && _cursor.isNull(_cursorIndexOfName))) {
final long _tmpId;
_tmpId = _cursor.getLong(_cursorIndexOfId);
final String _tmpName;
_tmpName = _cursor.getString(_cursorIndexOfName);
_tmpUser = new User(_tmpId,_tmpName);
} else {
_tmpUser = null;
}
final Office _tmpOffice;
if (! (_cursor.isNull(_cursorIndexOfId_1) && _cursor.isNull(_cursorIndexOfAddress))) {
final long _tmpId_1;
_tmpId_1 = _cursor.getLong(_cursorIndexOfId_1);
final String _tmpAddress;
_tmpAddress = _cursor.getString(_cursorIndexOfAddress);
_tmpOffice = new Office(_tmpId_1,_tmpAddress);
} else {
_tmpOffice = null;
}
final Places _tmpPlaces;
if (! (_cursor.isNull(_cursorIndexOfId_2) && _cursor.isNull(_cursorIndexOfName_1))) {
final long _tmpId_2;
_tmpId_2 = _cursor.getLong(_cursorIndexOfId_2);
final String _tmpName_1;
_tmpName_1 = _cursor.getString(_cursorIndexOfName_1);
_tmpPlaces = new Places(_tmpId_2,_tmpName_1);
} else {
_tmpPlaces = null;
}
_result = new UserOfficePlacesCombined();
_result.setUser(_tmpUser);
_result.setOffice(_tmpOffice);
_result.setPlaces(_tmpPlaces);
} else {
_result = null;
}
return _result;
} finally {
_cursor.close();
_statement.release();
}
}
The critical lines are the ones like :-
final int _cursorIndexOfId = CursorUtil.getColumnIndexOrThrow(_cursor, "userid")
This is used to search for the column's names in the Cursor (aka result set) and return the offset to the column, the index then being used to get the actual value from the Cursor.
In your scenario the result set will include some like
rUserId rUserId rUserId*
Which one should it use for which? You may know/understand that first is ruser.rUserId, and that the second is account.rUserId and that the third is accountgroup.rUserId but Room, as it stands, will not know when generating the code. So in all 3 instances when getColumnIndex("rUserId") is used, it will return either 0 (the first) it breaks out of the loop, or 2 if it continues rather than breaks out of the loop (I believe it doesn't break out of the loop).