IssueTracker

Actually that was an experimental API we thought about removing a couple of times but decided to keep for now.
About 1-1, it is actually better to write them as join queries. Is there a case where join is not feasible?

The issue with a JOIN is not feasibility. I suspect that can be done, albeit at the cost of yet another non-entity POJO.

Let's flip it around: given your characterization of the origins of @Relation ("an experimental API") and its possible lame-duck status ("decided to keep for now"), should we be using it?

How would I load all the relation objects immediately without a relation? I see in your documentation that you have a query like "@Query("SELECT user.name AS userName, pet.name AS petName "
          + "FROM user, pet "
          + "WHERE user.id = pet.user_id")
   public LiveData<List<UserPet>> loadUserAndPetNames();", but how would you load complete objects with it, not just certain fields? I tried using something like "SELECT * , * as comments FROM users, comments", but it was throwing an error?

I'm interested in this feature along with the ability to use a projection like "[expression] AS [column-name]" and map to a single value. Additionally, since the @Relation annotation is never mentioned on the Room page, will it be maintained?

It will be maintained, and added to the Room page.

Project: platform/frameworks/support
Branch: androidx-master-dev

commit c5dad6522e6c20bbd5ca8e53b8406a94cb84cfe8
Author: Daniel Santiago Rivera <danysantiago@google.com>
Date:   Wed Jun 12 16:43:53 2019

    Allow single-value @Relation field.
   
    This change lifts the restriction of @Relation annotated fields needing
    to be List or Set allowing for a more seamless representation of a
    one-to-one relationship.
   
    Changes in the generated code is so that it is not always assumed that
    the relation field is a collection, thus not needing a temporary
    collection when collecting the relating objects.
   
    Bug: 62905145
    Test: ./gradlew room:room-compiler:test \
                    room:integration-tests:room-testapp:cC
   
    Change-Id: I462d59e89b55c43842df43cb84c1aeca0b2b743e

M       room/common/src/main/java/androidx/room/Relation.java
M       room/compiler/src/main/kotlin/androidx/room/processor/PojoProcessor.kt
M       room/compiler/src/main/kotlin/androidx/room/processor/ProcessorErrors.kt
M       room/compiler/src/main/kotlin/androidx/room/vo/RelationCollector.kt
M       room/compiler/src/main/kotlin/androidx/room/writer/RelationCollectorMethodWriter.kt
M       room/compiler/src/test/kotlin/androidx/room/processor/PojoProcessorTest.kt
M       room/compiler/src/test/kotlin/androidx/room/processor/QueryMethodProcessorTest.kt
M       room/integration-tests/testapp/src/androidTest/java/androidx/room/integration/testapp/dao/PetDao.java
M       room/integration-tests/testapp/src/androidTest/java/androidx/room/integration/testapp/test/PojoWithRelationTest.java
A       room/integration-tests/testapp/src/androidTest/java/androidx/room/integration/testapp/vo/PetAndOwner.java

https://android-review.googlesource.com/982373
https://goto.google.com/android-sha1/c5dad6522e6c20bbd5ca8e53b8406a94cb84cfe8