Exercise 2: Two Collections, Two Correct ID Rules — Possible Solution
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WHY THE ITEMS COLLECTION AVOIDS BARCODE-AS-ID
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Per Chapter 2, the items collection tracks one document per purchased instance of an item - the same product (and therefore the same barcode) can be bought and tracked multiple times, each with its own separate expiry date and lifecycle. Using the barcode as the document ID would collide the moment the same product was purchased a second time.

WHY THE BARCODECACHE COLLECTION USES BARCODE-AS-ID CORRECTLY
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barcodeCache tracks one document per product, not per purchase - there is only ever one canonical name/category for a given barcode, regardless of how many times that product gets bought and scanned. Since there's genuinely only one "instance" per barcode in this collection's own purpose, keying it directly by barcode causes no collision and is exactly the right design.

WHAT'S DIFFERENT BETWEEN THE TWO COLLECTIONS
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The difference is what each collection's document actually represents: items represents a many-per-barcode relationship (many purchases can share one barcode), while barcodeCache represents a strictly one-per-barcode relationship (one canonical product record per barcode). The same "should the ID be the barcode?" question has two different correct answers because the underlying cardinality of what's being modeled is different in each collection.

WHY THIS WORKS AS AN ANSWER
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It correctly explains why items must avoid barcode-as-ID (many purchases can share one barcode) and why barcodeCache can safely use it (exactly one record per barcode), tying both back to the different cardinality each collection actually represents rather than treating the two rules as contradictory.