add unique constraint in room database to multiple column

Question

I have one entity in room

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

sql syntax of this entity is as below

CREATE TABLE `LabelOfTask` (     `_id` INTEGER PRIMARY KEY AUTOINCREMENT,      `labelId` INTEGER,      `taskId` INTEGER,      FOREIGN KEY(`labelId`) REFERENCES `Label`(`_id`) ON UPDATE NO ACTION ON DELETE CASCADE ,      FOREIGN KEY(`taskId`) REFERENCES `Task`(`_id`) ON UPDATE NO ACTION ON DELETE CASCADE  ); 

but what change or annotation I need to add in entity class if I want to append below constraint to the auto generated sql schema of the table

unique (labelId, taskId) 

Ultimately I want to make combination of labelId and taskId unique in a table(or entity of room) using room library.

Answer 1

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; } 

Answer 2

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

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