EntityHydrator fails to nested relations

[heiko-r]

Describe the bug
I'm trying to populate the 'product.facetValues.facet' relation of a ProductVariant. I obtain the ProductVariants from ProductVariantService.findByIds(), so they don't have the product relation yet.

There seem to be two related bugs in EntityHydrator.getMissingRelations:

• Line 151 doesn't take the possibility of an empty array into account, setting entity to undefined, which throws an error when trying to hydrate a further nested relation.
• After Line 160, entity is not updated, so that the next part of the relation path is checked against the wrong entity.

To Reproduce
Steps to reproduce the behavior:

1. Create a Product with some facet values, and a ProductVariant without facets
2. Use this.entityHydrator.hydrate(ctx, variant, { relations: ['product.facetValues.facet'] }):
   -> It throws the error "Cannot read property 'facet' of undefined" due to:
   • The missing product relation is correctly added to missingRelations, however entity is not updated
   • facetValues is confirmed to exist on the ProductVariant (not ProductVariant.product!), but in this case it's an empty array, so the new value for entity becomes undefined
3. Use this.entityHydrator.hydrate(ctx, variant, { relations: ['product.facetValues'] }):
   • It only hydrates the product relation, because facetValues is checked against the ProductVariant, not the Product.
4. It works when hydrating the nested relations level-by-level:

   this.entityHydrator.hydrate(ctx, variant, { relations: ['product'] });
   this.entityHydrator.hydrate(ctx, variant, { relations: ['product.facetValues'] });
   this.entityHydrator.hydrate(ctx, variant, { relations: ['product.facetValues.facet'] });

Expected behavior
EntityHydrator should work with nested relations.

Environment (please complete the following information):

• @vendure/core version: 1.3.0
• Nodejs version: 14.17.0
• Database (mysql/postgres etc): PostgreSQL

[michaelbromley]

Thanks for the detailed report! Will get this fixed in the next patch.