StringArrayFieldConverter.java

public class StringArrayFieldConverter implements FieldConverter<String[]> {
    @Override
    public String[] fromCursorValue(Cursor cursor, int columnIndex) {
        return cursor.getString(columnIndex).split(",");
    }

    @Override
    public void toContentValue(String[] value, String key, ContentValues values) {
    }

    @Override
    public ColumnType getColumnType() {
        return ColumnType.JOIN;
    }
}

Test.java

    public void testJoinEntity() {
        final Cupboard cupboard = new CupboardBuilder().registerEntityConverterFactory(new EntityConverterFactory() {
            @Override
            public <T> EntityConverter<T> create(Cupboard cupboard, Class<T> type) {
                if (type == TestJoinEntity.class) {
                    return (EntityConverter<T>) new TestJoinEntityConverter();
                }
                return null;
            }
        }).build();
        SQLiteOpenHelper helper = new SQLiteOpenHelper(getContext(), "test_ls.db", null, 1) {
            @Override
            public void onCreate(SQLiteDatabase db) {
                cupboard.withDatabase(db).createTables();
            }

            @Override
            public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

            }
        };
        SQLiteDatabase db = helper.getWritableDatabase();
        cupboard.register(TestJoinEntity.class);
        cupboard.withDatabase(db).createTables();
        db.execSQL("create table ids (_id INTEGER PRIMARY KEY AUTOINCREMENT, value TEXT)");
        ContentValues values = new ContentValues();
        values.put("value", "test1");
        db.insert("ids", null, values);
        values.put("value", "test2");
        db.insert("ids", null, values);
        TestJoinEntity entity = new TestJoinEntity();
        cupboard.withDatabase(db).put(entity);
        Cursor cursor = db.rawQuery("select e._id as _id, group_concat(j.value) as names from testjoinentity e, ids j", null);
        assertEquals(1, cursor.getCount());
        entity = cupboard.withCursor(cursor).get(TestJoinEntity.class);
        assertNotNull(entity.names);
    }

    public void testJoinEntityWithFieldConverter() {
        final Cupboard cupboard = new CupboardBuilder().registerFieldConverter(String[].class, new StringArrayFieldConverter()).build();
        SQLiteOpenHelper helper = new SQLiteOpenHelper(getContext(), "test_ls.db", null, 1) {
            @Override
            public void onCreate(SQLiteDatabase db) {
                cupboard.withDatabase(db).createTables();
            }

            @Override
            public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

            }
        };
        SQLiteDatabase db = helper.getWritableDatabase();
        cupboard.register(TestJoinEntity.class);
        cupboard.withDatabase(db).createTables();
        db.execSQL("create table ids (_id INTEGER PRIMARY KEY AUTOINCREMENT, value TEXT)");
        ContentValues values = new ContentValues();
        values.put("value", "test1");
        db.insert("ids", null, values);
        values.put("value", "test2");
        db.insert("ids", null, values);
        TestJoinEntity entity = new TestJoinEntity();
        cupboard.withDatabase(db).put(entity);
        Cursor cursor = db.rawQuery("select e._id as _id, group_concat(j.value) as names from testjoinentity e, ids j", null);
        assertEquals(1, cursor.getCount());
        entity = cupboard.withCursor(cursor).get(TestJoinEntity.class);
        assertNotNull(entity.names);
    }

TestJoinEntity.java

public class TestJoinEntity {
    public Long _id;
    public String[] names;
}

TestJoinEntityConverter.java

public class TestJoinEntityConverter implements EntityConverter<TestJoinEntity> {
    @Override
    public TestJoinEntity fromCursor(Cursor cursor) {
        TestJoinEntity entity = new TestJoinEntity();
        entity._id = cursor.getLong(0);
        String names = cursor.getString(1);
        if (names != null) {
            entity.names = names.split(",");
        }
        return entity;
    }

    @Override
    public void toValues(TestJoinEntity object, ContentValues values) {
        if (object._id != null) {
            values.put(BaseColumns._ID, object._id);
        }
    }

    @Override
    public List<Column> getColumns() {
        return Arrays.asList(new Column("_id", ColumnType.INTEGER), new Column("names", ColumnType.JOIN));
    }

    @Override
    public void setId(Long id, TestJoinEntity instance) {
        instance._id = id;
    }

    @Override
    public Long getId(TestJoinEntity instance) {
        return instance._id;
    }

    @Override
    public String getTable() {
        return TestJoinEntity.class.getSimpleName();
    }
}