Quick glance that demonstrates generating a proper Room object model including an M:N junction table. This could probably be done more efficiently. Ideally @relation could walk through ForeignKey avoiding all this nonsense. M:N "List<String>" is GROUP_CONCAT to reduce the result to single entities, then the keyword is split with a TypeConverter.

Dao.java

@Dao
public abstract class MetadataDao {
	@Query("SELECT id, name, type, height, width, orientation,  " +
				"(SELECT GROUP_CONCAT(name) " +
					"FROM meta_subject_junction " +
					"JOIN xmp_subject " +
					"ON xmp_subject.id = meta_subject_junction.subjectId " +
					"WHERE meta_subject_junction.metaId = meta.id) AS keywords, " +
				"(SELECT documentUri " +
					"FROM image_parent " +
					"WHERE meta.parentId = image_parent.id ) AS parentUri " +
			"FROM meta")
	abstract LiveData<List<MetadataResult>> getImages();
}

MetadataResult.java

@TypeConverters({MetadataResult.class})
public class MetadataResult extends MetadataEntity {
    public List<String> keywords;
    public String parentUri;

    @TypeConverter
    public List<String> fromGroupConcat(String keywords) {
        return Arrays.asList(keywords.split(","));
    }
}

SubjectEntity.java

@Entity(tableName = "xmp_subject",
		indices = @Index(value = "id"))
public class SubjectEntity extends PathEntity
{
	SubjectEntity() {}
	SubjectEntity(String path, int depth, long parent, String name, Long recent)
	{
		super(path, depth, parent);
		this.name = name;
		this.recent = recent;
	}

	SubjectEntity(Long id, String path, int depth, long parent, String name, Long recent)
	{
		super(id, path, depth, parent);
		this.name = name;
		this.recent = recent;
	}

	public String name;
	public Long recent;
}

SubjectJunction.java

@Entity(
	tableName = "meta_subject_junction",
	primaryKeys = {"metaId", "subjectId"},
	indices = {
		@Index(value = "subjectId"),
		@Index(value = "metaId")},
	foreignKeys = {
		@ForeignKey(
			entity = MetadataEntity.class,
			parentColumns = "id",
			childColumns = "metaId",
			onDelete = CASCADE),
		@ForeignKey(
			entity = SubjectEntity.class,
			parentColumns = "id",
			childColumns = "subjectId",
			onDelete = CASCADE)})
public class SubjectJunction {
	public SubjectJunction(@NonNull Long metaId, @NonNull Long subjectId) {
		this.metaId = metaId;
		this.subjectId = subjectId;
	}

	@NonNull
	public Long metaId;

	@NonNull
	public Long subjectId;
}