Empty hasOne relation should return null instead of {} when serialized

[ricardograca]

• Related Issues: Collection returns empty object for null relations #1771, Related models are empty objects instead of null when not found #1299, hasOne relations return empty instances when no model found #753
• Previous PRs: Do not load belongsTo related model when FK is null #1563

Introduction

When fetching a model that has a hasOne relation and that relation doesn't exist the relation should be null when serializing the model. Currently it's an empty object {}.

Motivation

This makes the hasOne relationship behave like belongsTo which also exhibited this same unexpected behavior until it was changed with PR #1563. All other relation types continue working the same way, so a hasMany without any records is still an empty array [] when serialized.

This should make it easier to determine if a hasOne relation exists or not since after serialization you can just write:

if (model.singleRelation) { ... }

instead of:

if (model.singleRelation && model.singeRelation.id) { ... }

Proposed solution

This changes the eager pairing code to ensure that the relation creation part is skipped if the response from the SELECT query that tries to fetch the related data is empty. The eager pairing would run pushModels() which would always create relation data on the parent model, even when there is no data, resulting in the empty object that was present before this change.

This is a behavior similar to the belongsTo relation, except that that one can be determined much earlier in the eager loading code chain.

Fixes #1771.

Current PR Issues

This is a breaking change, but it shouldn't cause too much trouble since any existing conditions, as in the example above, will still continue working. Other use cases that attempt to add or read properties directly from the serialized output will break.

[OlivierKamers]

Is it correct that the behaviour is to actually omit the relation from the serialised model (ie making it undefined) instead of setting it to null?

[ricardograca]

Are you asking if it can be changed to that or are you saying that it's actually undefined currently?

[OlivierKamers]

I noticed that it's actually undefined currently. So I was wondering if I might be doing something wrong or if it's supposed to behave like that.

[ricardograca]

Good point. Looks like it is not even defining the relation key at all. This only matters if you explicitly look for null or check if the key is present, but it should be consistent indeed. Can you open a new issue?

[OlivierKamers]

Done, thank you for the quick response.