Tick Nike, and you can't add Adidas. Facets done wrong.

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You tick Nike in the brand filter. Now you want Adidas too. It's gone from the list.

Search running shoes. 2,847 results. Beside them, counts. Nike, 892. Adidas, 567. New Balance, 234. Brooks, 189. Each count is an aggregation, computed alongside the search.

Counts update as filters apply. Tick Nike, and colour and price narrow to Nike. But here's the catch. Narrow the brand facet too, and it shows only Nike.

So split the work. The query holds the text, and in stock. Your selections go in a post filter. It narrows the hits you see. But not the aggregations. Those are computed separately. Each facet applies every selection except its own. Brand ignores brand.

So with Nike ticked, the brand facet counts as if no brand were chosen. Adidas is still there. Tick it. Multi-select just works. Colour and price still narrow to your brands. Just not the brand facet itself.

Facets are how users who don't know what they want explore. Every facet counts with every filter except its own.

Full breakdown in the lecture.