how to add prefix in roomDatabase?

I have a problem in my roomDatabase.

I searched on google and found some solutions but I can't solve my problem.

Can anyone help me?

Note : I want to use Object Oriented in my project.

Error :

Multiple fields have the same columnName: class_id. Field names: classId, classId.

ClassEntry :

@Entity(tableName = "class")

public class ClassEntry {

@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = COLUMN_CLASS_ID)
private int classId;

@ColumnInfo(name = "name")
private String className;
@ColumnInfo(name = "day")
private String classDay;

public ClassEntry(int classId, String className, String classDay) {
    this.classId = classId;
    this.className = className;
    this.classDay = classDay;
}

@Ignore
public ClassEntry(int classId) {
    this.classId = classId;
}

StudentEntry :

@Entity(tableName = "student",
    primaryKeys = {COLUMN_CLASS_ID, COLUMN_STUDENT_ID},
    foreignKeys = {@ForeignKey(
            entity = ClassEntry.class,
            parentColumns = COLUMN_CLASS_ID,
            childColumns = COLUMN_CLASS_ID,
            onDelete = CASCADE)})
public class StudentEntry extends ClassEntry{

@ColumnInfo(name = COLUMN_CLASS_ID)
private int classId;
@NonNull
@ColumnInfo(name = COLUMN_STUDENT_ID)
private String studentId;
@ColumnInfo(name = "first_name")
private String firstName;
@ColumnInfo(name = "last_name")
private String lastName;

public StudentEntry(int classId, @NotNull String studentId, String firstName, String lastName) {
    super(classId);
    this.studentId = studentId;
    this.firstName = firstName;
    this.lastName = lastName;
}

Your issue is that you are extending the StudentEntry class with the ClassEntry class. This is not how you manage related data with Room. You are effectively saying you want a table inside. Rather as you want a one (class) to many (students) relationship.

As such you want an Object that will contain a ClassEntry and the respective StudentEntries (0 or more). For this you create a normal POJO class embedding the Parent table and relating the child class(es).

So your ClassEntry class is fine as it is. However your StudentEntry class could be :-

@Entity(tableName = "student",
        primaryKeys = {COLUMN_CLASS_ID, COLUMN_STUDENT_ID},
        foreignKeys = {@ForeignKey(
                entity = ClassEntry.class,
                parentColumns = COLUMN_CLASS_ID,
                childColumns = COLUMN_CLASS_ID,
                onDelete = CASCADE)})
public class StudentEntry /* extends ClassEntry <<<<<<<<<< COMMENTED OUT */ {

    @ColumnInfo(name = COLUMN_CLASS_ID)
    private int classId;
    @NonNull
    @ColumnInfo(name = COLUMN_STUDENT_ID)
    private String studentId;
    @ColumnInfo(name = "first_name")
    private String firstName;
    @ColumnInfo(name = "last_name")
    private String lastName;

    public StudentEntry(int classId, @NonNull String studentId, String firstName, String lastName) {
        /* super(classId); <<<<<<<<<< COMMENTED OUT */
        this.classId = classId; //<<<<<<<<<< ADDED
        this.studentId = studentId;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @NonNull
    public String getStudentId() {
        return studentId;
    }

    public void setStudentId(@NonNull String studentId) {
        this.studentId = studentId;
    }

    public int getClassId() {
        return classId;
    }

    public void setClassId(int classId) {
        this.classId = classId;
    }
}

• i.e. it doesn't extend the ClassEntry and therefore doesn't call the Class Entry super. see comments.
• note redundant getters and setters removed

Then you add another POJO class such as :-

class ClassEntryWithStudentEntrys {

    @Embedded
    ClassEntry classEntry;
    @Relation(entity = StudentEntry.class,parentColumn = COLUMN_CLASS_ID,entityColumn = COLUMN_CLASS_ID)
    List<StudentEntry> studentEntryList;
}

• @Embedded includes the following class
• @Relation defines the relationship

The Dao's could be :-

@Dao
interface AllDao {

    @Insert
    long insert(ClassEntry classEntry);
    @Insert
    long insert(StudentEntry studentEntry);
    @Transaction
    @Query("SELECT * FROM class")
    List<ClassEntryWithStudentEntrys> getAllClassEntriesWithStudentEntries();

}

• the Last one utilising the POJO with the Embedded and Relation annotations.

Working Example using the above

Consider the following code, this adds 2 Classes Class1 and Class2. Class1 has 2 students added, Class 2 has 3 students added. Finally the ClassEntriesWithStudnetEntries are extracted using the getAllClassEntriesWithStudentEntries Query:-

    dao = db.getAllDao();

    long class1 = dao.insert(new ClassEntry(0,"Class1","Mon"));
    dao.insert(new StudentEntry((int)class1,"student1","Mary","Smith"));
    dao.insert(new StudentEntry((int) class1,"student2","Fred","Bloggs"));
    long class2 = dao.insert(new ClassEntry(0,"Class2","Tue"));
    dao.insert(new StudentEntry((int) class2,"student3","Sarah","Tomms"));
    dao.insert(new StudentEntry((int)class2,"student4","Alan","Richardson"));
    dao.insert(new StudentEntry((int)class2,"student5","Jayne","Lockyer"));

    List<ClassEntryWithStudentEntrys> classWithStudentsList = dao.getAllClassEntriesWithStudentEntries();
    for (ClassEntryWithStudentEntrys c: classWithStudentsList) {
        Log.d("CLASSSTUDENTINFO","Class is " + c.classEntry.getClassName() + " on Day " + c.classEntry.getClassDay() + " ID is " + c.classEntry.getClassId());
        for (StudentEntry s: c.studentEntryList) {
            Log.d("CLASSSTUDENTINFO","\tStudent is " + s.getFirstName() + "," + s.getLastName() + " Student ID is " + s.getStudentId());
        }
    }

Result as output to the log

D/CLASSSTUDENTINFO: Class is Class1 on Day Mon ID is 1
D/CLASSSTUDENTINFO:     Student is Mary,Smith Student ID is student1
D/CLASSSTUDENTINFO:     Student is Fred,Bloggs Student ID is student2
D/CLASSSTUDENTINFO: Class is Class2 on Day Tue ID is 2
D/CLASSSTUDENTINFO:     Student is Sarah,Tomms Student ID is student3
D/CLASSSTUDENTINFO:     Student is Alan,Richardson Student ID is student4
D/CLASSSTUDENTINFO:     Student is Jayne,Lockyer Student ID is student5

• Note the above is just a demo, it is not intended to be rerun.