add unique constraint in room database to multiple column

A plain UNIQUE constraint on a column, other than via an index, is not supported.

You can enforce this uniqueness property by setting the unique property of an @Index annotation to true. The following code sample (Java) prevents a table from having two rows that contain the same set of values for the firstName and lastName columns:

@Entity(indices = {@Index(value = {"first_name", "last_name"},        unique = true)})class User {    @PrimaryKey    public int id;    @ColumnInfo(name = "first_name")    public String firstName;    @ColumnInfo(name = "last_name")    public String lastName;    @Ignore    Bitmap picture;}

The Kotlin equivalent of the annotation is given below:

@Entity(indices = [Index(value = ["first_name", "last_name"], unique = true)])

In your code you can do the following changes to have UNIQUE constraints

@Entity(foreignKeys ={        @ForeignKey(entity = Label.class, parentColumns = "_id", childColumns = "labelId", onDelete = CASCADE),        @ForeignKey(entity = Task.class, parentColumns = "_id", childColumns = "taskId", onDelete = CASCADE)},        indices = {@Index(value = {"labelId", "taskId"},                unique = true)})public class LabelOfTask extends Data{    @ColumnInfo(name = "labelId")    private Integer labelId;    @ColumnInfo(name = "taskId")    private Integer taskId;}

If you wonder to make a single column to be unique, only need to write

@Entity(indices = [Index(value = ["name"], unique = true)])

For a single column Uniqueness

@Entity(indices = {@Index(value = {"first_name"},unique = true)})

For Multiple column Uniqueness

@Entity(indices = {@Index(value = {"first_name", "last_name"},unique = true)})