Error when joining with a computed value

[duailibe]

Right now when trying to do a join using a computed value in the comparison, we get an error.

Reproduction

Ran this in packages/db/tests/query/join.test.ts:

type Product = {
  productId: number;
  region: 'A' | 'B' | 'C';
  sku: string;
  title: string;
}

type ProductInventory = {
  inventoryId: number;
  region: 'A' | 'B' | 'C';
  sku: string;
  quantity: number;
};

test('join with concat', () => {
  const productsCollection = createCollection(
    mockSyncCollectionOptions<Product>({
      id: `test-products-join`,
      getKey: (product) => product.productId,
      initialData: [
        { productId: 1, region: 'A', sku: 'sku1', title: 'A1' },
        { productId: 2, region: 'B', sku: 'sku1', title: 'B1' },
        { productId: 3, region: 'C', sku: 'sku2', title: 'C2' },
        { productId: 4, region: 'A', sku: 'sku2', title: 'A2' },
      ],
    })
  );
  const inventoriesCollection = createCollection(
    mockSyncCollectionOptions<ProductInventory>({
      id: `test-inventories-join`,
      getKey: (inventory) => inventory.inventoryId,
      initialData: [
        { inventoryId: 1, region: 'A', sku: 'sku1', quantity: 10 },
        { inventoryId: 3, region: 'C', sku: 'sku2', quantity: 30 },
      ],
    })
  );

  const joinQuery = createLiveQueryCollection({
    startSync: true,
    query: (q) =>
      q
        .from({ product: productsCollection })
        .join(
          { inventory: inventoriesCollection },
          ({ product, inventory }) =>
            eq(concat(product.region, product.sku),
              concat(inventory.region, inventory.sku)),
        )
  });

  expect(joinQuery.toArray).toHaveLength(2);
});

And it fails with:

TypeError: Cannot read properties of undefined (reading 'length')
 ❯ followRef src/query/compiler/index.ts:4147:16
    512|     }
    513|   }
    514| }
       |  ^
    515|
 ❯ processJoin src/query/compiler/joins.ts:425:29
 ❯ processJoins src/query/compiler/joins.ts:139:3
 ❯ compileQuery src/query/compiler/index.ts:198:5
 ❯ CollectionConfigBuilder.compileBasePipeline src/query/live/collection-config-builder.ts:337:9
 ❯ new CollectionConfigBuilder src/query/live/collection-config-builder.ts:151:5
 ❯ liveQueryCollectionOptions src/query/live-query-collection.ts:269:60
 ❯ createLiveQueryCollection src/query/live-query-collection.ts:287:46