Paragraph prior to the (repeated) assert_keys/3 on page 309 says:

We can chain further reducers and assertions onto our pipeline to test the final scenario. The assign_products_with_average_ratings/1 function should populate the socket with the correct product ratings, given the provided filters, like this:

On the previous page (308), the second snippet already included assign_products_with_average_ratings/1 in the pipeline. This snippet on page 309, however, shows the assert_keys/3 implementation, as opposed to (an updated?) snippet that includes assign_products_with_average_ratings/1. Probably removing this snippet and rephrase the paragraph prior to and following it would be helpful.