The cache fetch code is all wrong. The parameter to fetch is a string that identifies the data you want. Your code is trying to use the same string for every user, so they would all see the same postings that would be saved by the first call of this method.
In my example below I used the user id and a string 'postings' to indicate all postings for a specific user.
It is incorrect to assign @postings inside the fetch block, the result of the block (the query result) is saved to @postings
Finally, ActiveRecord delays making the actual database call until absolutely necessary. The .all call at the end of the query will return the data, and the data is what you want cached, not the configuration data used to create a query.
Here is the correct code:
@postings = Rails.cache.fetch("#{@user.id}:postings") do
@user.postings.includes(:category).all
end