Where the presence of an object is being validated and that object comes from an *association*, we should use `validates :object, presence: true` instead of `validates :object_id, presence: true`.
This does not apply in the same way to validations on uniqueness of certain attributes, such as `validates :object_id, uniqueness...`
In the line below we filter them out in Ruby so it's a waste of
resources. The fundamental difference is that `#includes` and
`#references` results in LEFT JOINs, whereas `#joins` results in INNER
JOIN, and because there's a default scope on `deleted_at IS NULL`, these
are not included in the result set.
This however, requires us to move away from the current algorithm but
unfortunately we can't refactor it completely yet.
Before:
```sql
SELECT *
FROM "variant_overrides"
LEFT OUTER
JOIN "spree_variants"
ON "spree_variants"."id" = "variant_overrides"."variant_id"
AND "spree_variants"."deleted_at" IS NULL
LEFT OUTER
JOIN "spree_products"
ON "spree_products"."id" = "spree_variants"."product_id"
AND "spree_products"."deleted_at" IS NULL
WHERE "variant_overrides"."permission_revoked_at" IS NULL
AND "variant_overrides"."hub_id" IN (
SELECT "enterprises"."id"
FROM "enterprises"
INNER
JOIN "enterprise_roles"
ON "enterprise_roles"."enterprise_id" = "enterprises"."id"
WHERE (enterprise_roles.user_id = ?)
AND (sells != 'none')
ORDER BY name)
```
After:
```sql
SELECT "variant_overrides".*
FROM "variant_overrides"
INNER
JOIN "spree_variants"
ON "spree_variants"."id" = "variant_overrides"."variant_id"
AND "spree_variants"."deleted_at" IS NULL
INNER
JOIN "spree_products"
ON "spree_products"."id" = "spree_variants"."product_id"
AND "spree_products"."deleted_at" IS NULL
WHERE "variant_overrides"."permission_revoked_at" IS NULL
AND "variant_overrides"."hub_id" IN (
SELECT "enterprises"."id"
FROM "enterprises"
INNER
JOIN "enterprise_roles"
ON "enterprise_roles"."enterprise_id" = "enterprises"."id"
WHERE (enterprise_roles.user_id = ?)
AND (sells != 'none')
ORDER BY name)
```
This is covered in the test suite by
spec/controllers/admin/variant_overrides_controller_spec.rb:72. It keeps
passing so we're good to go.
