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A common example of a common eCommerce website with two collections for the products and the categories. Each product belongs to a certain category like "shoes", "hats", and "pants" – and we want to refer to a corresponding collection from each product. We do not want to include the actual name of the category into a product's record since it will complicate renaming categories. Instead of the default MongoDB random sequence of letters and numbers, we want the corresponding category's name.